Andrew, I realize that many articles have been written on alternate methods than KOPS. But as a former athlete, and later team trainer for 2 different teams, I have to stick to it's logic, which is anchored in sound biomechanics. Within KOPS, certain riders have different riding styles (which we can argue is less efficient or not), and which will call for deviations on that. But you'll have deviations on no matter what method you use. STA can be changed to fit a rider, no problem, as it doesn't affect steering. (it does affect weight distribution, but ergonomics should come first, as any adjustments to weight distribution can be fine-tuned with the chainstay length.

YES, there WERE criterium bikes. But they were not the ones you think. They weren't necessarily the (insert brand name) labelled "criterium" any more than the department store bike labelled "racing bike" actually was one. My Rino was a criterium geometry as I have described, but it was labeled... I'll come back and tell you if I remember (or can read it off of an old photograph when I find it) - maybe Corsa, or Competition, something like that. And I have had friends who had bikes with a Crit geometry too. Wish I could remember the makers. Anyways, just because you did not experience or were not exposed to something, doesn't make it impossible to have existed.

You choose your words carefully: "feel more efficient" isn't the same as "being more efficient". If proven wrong, you can argue that you didn't commit, but was just in the mood for arguing. ;p I've thought it through, and I don't think any flex is more efficient. Flex, done the right way, becomes part of the suspension, as slight as it is on a bike. Done the wrong way, and you have cable tugs that are sufficient to switch gears with your rear derailleur when going uphill, as I had experienced on a Columbus "Special" ride (beats me if in this case "Special" referred to SL or SP - probably a mix of the two, as it was a 60cm, size at which they start to use SP in SL-class frames).

Let me tell you about downtube flex, and most modern bike designs. Do you remember Bicycling Magazine? And their spinoff, I believe it was called Bicycling Tech, or something like that. Their way of becoming famous (which didn't prevent it from being a flop because of too little readership), was them building the "Tarantula". A large, complicated frame jig, that was designed to test the flex of different bicycles. It anchored the bike at the dropouts and the headtube (perhaps also at the seatpost? - I can't visually recall that), and they applied a force to the crankarms, externally. It also makes the presumption that you are pulling with your arms, to push on the pedals.
And it was from this that they concluded the flex was coming from the downtube. And everyone believed them. No one ever questioned their methodology, whether they were doing it right.
I can go up hills, no-hands. The steepest ones I have to rest my hands on the bars, and shift my weight forward, otherwise the front wheel goes up off the ground, but you get my point. When you are able to properly pedal, exerting force 360°, it's your saddle, anchored between buttocks (from gravity) and against your inner thigh, that is the resisting other end of the lever. The downtube has a role of stabilizing the bike, preventing it from twisting at speed, and assisting those using their arms too much in order to compensate bad pedaling style. You can watch "the cycling network" videos on youtube where they teach you how to do it wrong. But there is so much length to be twisted there, under torsion, that a cast bottom bracket with an OS seat tube (or with a longer butted section at the base), can better to overcome the BB sway. And BTW, Colnago has got it wrong. Downtube needs torsional rigidity. Their crimped Gilco tubing, co-designed by Ferrari who know nothing about bikes, only results in a ride that transmits more vibrations to the rider, with a downtube that has worse torsional rigidity than a round shape.
And there you have it. I don't want to get into deep discussions publicly, on how it should have been done, because there are several elements about bike design that I wish to keep a trade secret, once I start building in CF. So I'll just leave it at that.

"Larger tires result in slowing down a bike's snappiness is a well known method of taming a bike." THAT, if you re-read yourself and stop to think for a moment, is silly, even by your own admission. Come-on! Surely you don't actually believe that. It's merely a symptom of cheaper tires. But I never said I put cheaper tires on my bike! They are Continentals. And the weight difference between 25c and 28c, within that brand for the same model, can be less than what you will see between differing brands for the same tire size. My 28c Contis weigh about the same as the 24Cs I have on my other bike. I stand by my statement, that it's the slight increase in trail, combined with better weight distribution, that solved the problem of the front wobble at moderate speed. BTW, I learned something from skateboarding to which few in the cycling industry are privileged: going downhill on a skateboard, speed wobbles become an extreme condition, which are easily resolved by shifting your weight onto the front truck. 100+km/h is all of a sudden no longer a problem. Moving the back wheel out (or shortening the top tube), are ways of changing weight distribution. Of course there are trade-offs, affecting handling, bike for for some riders, and braking.