The criteria you have listed (for as much objectivity as you've offered) are thinness and hardness.
Alone these don't mean what you think they do.
They mean exactly what I know they do
For a given thickness, you can have many different potential taper angles depending how "tall" the blade is and how far up the blade its' sharpened. And many aren't tapered at all any more. A Victorinox paring knife is far thinner than the chef's knives you're talking about for example, but is also "shorter". So you have less distance to achieve the taper towards the edge. I've not measured them but I suspect the paring knife in question doesn't taper at all really, they just ground an edge on a thin hardish piece of steel. Sufficient for a paring knife.
A victorinox knife has one of the two magical characteristics which is a thin blade. It is missing the hardness part, however. The reason Victorinox knives are so popular other than their price is that they cut better than knives with thicker blades. Consider the razor blade. It is so thin that it will out cut any knife but its size and flexibility make it the wrong thing to replace kitchen cutlery. The thinner the blade the less resistance to cutting. Always. Can a kitchen knife be too thin? Of course. We will get into that.
The other element is the hardness of the steel. A 15 degree per bevel angle will cut better than a 20 degree per bevel angle. But if the steel isn't hard enough, the edge can fail requiring it to be ground to a more obtuse angle. It the blade edge fails or chips then the bevel angle is too acute for the steel and the application. Those angles are same regardless of the thickness of the blade
And many chef knives have a distal taper too, so thinness/thickness varies across the length of the blade.
So what do you mean by thinness. Which thinnesses are good and which are bad and why?
Hint, it depends on what you want the blade to do. Flexibility in a boning knife can be desirable, but it often depends on the user preference as you can get both flexible and rigid ones.
The right thickness is the amount necessary to ensure that the blade is rigid and straight. Any more thickness just adds more resistance to cutting. Yes, know there are flexible boning knives available. I don't own one. They are soft and require too much maintenance.
Thinner Japanese blades are often desribed as "whippy" for example, a negatively perceived characteristic for many. So it's not just about absolute thinness.
Hardness also isn't an absolute. Higher values lead to more chipping and brittleness in all steels. The degree varying with the metallurgy. But those are negative attributes so higher hardness isn't absolutely better, but a trade off of attributes related to the steel itself.
I've never heard knives referred to as whippy. So I wouldn't use the term often with your statement. Harder blades are more brittle for sure. They can accomodate more acute bevel angles but they can suffer chipping if the angles are too acute. The higher hardness is always better because it can accomodate a more acute bevel angle. Can a knife be too hard? Of course. A meat cleaver hardened to the level of a sushi knife will fail because of its application. These knives should be softer and have more obtuse angles.
Most of the big name brands are in the 58 hardness region, not that removed from the 61 and 62 of the Japanese steels. So you need to explain why those differences are meaningful.
You can't talk hardness meaningfully without a deep dive in to metallurgy and temper and heat treatment, hopefully with a cryo quench. You can take a lesser steel to a high hardness, but it just makes it chip prone. I have a gifted blade (I don't like) that claims it's AUS8 at 62 RC. It can't hold the edge it takes at that hardness if true. And it certainly doesn't behave like it's that hard either.
Hardness is meaningful because it determines to a large extent how acute the bevel angle can be as I said above. It has a secondary benefit in better edge retention. The difference between a blade tempered to RC58 and one hardened to RC62 is a lot. The RC58 blade requires a 20 degree per bevel angle while the RC62 blade may be able to handle a 12 degree per bevel angle. The more acute the angle the less resistance to cutting.
Heat treatment is a two step process. The hot steel is quenched to make the steel hard and then it is tempered to relax the steel to make it tougher (less brittle.) The tempering reduces quenched hardness but is important. AUS-8, like 440C generally tempers to RC56-58. I don't think it can be quenched to RC62 but I will take your word for it.
There are steels designed to be tempered to RC62. ZDP-189 is an example. It is not a good steel for a knife that will spend its life contacting a cutting board. AUS-8 would be better but would require a significantly more obtuse bevel angle. The stainless Japanese knives have steels appropriate for RC59-60.
I'm kind of interested that there has been as much discussion of handles. If you're using a pinch grip which most chef's use, there's not much difference between handles.
The best way to grip a kitchen knife is to "pinch" the blade just in front of the handle with the thumb and forefinger. The palm contacts the top of the handle and the other fingers prevent rotation. This is also the point at which the knife should balance. Balance is important for a commercial cook who uses a knife all day long because it is less tiring. It is less important for casual cooks like you and me.
You will find that most Japanese gyutos balance where they should while bolstered Euro chef knives are handle heavy. Japanese slicers are usually a little blade heavy but that is still less tiring than being handle heavy.
Buying a knife is, I think, about buying into a system within a given price point.
Will the owner sharpen it themself, or do they hire that out. If they're doing it themself, what gear do they have and is it appropriate to the knife they'll get.
For a lower budget, a Japanese blade and waterstones may not fit.
Why are they a system? If you were to inspect my knife block you would find that there aren't any knives there that share the same brand. I don't care that the knives were made by the same hands or their appearance. I care how they perform. I use the best knife I can for a particular application. About half of them are made from carbon steel instead of stainless steel.
Yes a cook should sharpen and maintain his or her own knives. They are the most important tools in the kitchen. Here is my advice. Do not use a powered sharpener. They remove more steel in the grinding process than necessary for sharpening and reduce the life span of the knives. I also recommend against bench stones unless one has the skill to use them properly. The best choice is what I call a guided manual sharpening system. While I am competent to use bench stones I use a guided system called the Edgepro because it is faster and I don't have to think about what I am doing.
The secret to sharpening a blade is maintaining a consistent angle throughout the process. The guided systems hold the blade at a consistent angle and that is why they are so effective. There are many guided system at many different prices. All of them will get the knives sharp.
Finally the cook should understand that the sharpening steel doesn't actually sharpen a blade. It straightens an edge that has become warped from contact with the cutting board. You still need to be able to grind away steel in order to sharpen a blade and the guided manual system is the ticket.
The traditional Japanese knives such as the yanagi have thick blades but with a single bevel. The inward side of the blade is flat and the outer side has the bevel which is fairly long because of the blade thickness. They are designed to be sharpened with bench stones. The angle consistency is automatic because you sharpen them by laying the bevel flat on the stone to grind and then you deal with the burr by laying the flat side of the blade on the stone. People who use traditional Japanese knives all use waterstones. It takes skill, though, to use them with Western pattern knives that have two bevels.
Japanese knives can get expensive but they don't have to be. I once sold a well used Masamoto gyuto on Ebay for $400 in an auction. It surprised me. On the other hand the MAC knives to which you referred earlier are pretty affordable and perform very well.
Let me know if I missed something.