My own take is that this used to matter, but doesn't any more. I have a number of old bikes, and a number of new ones.
All the new ones shift perfectly, and are easily stiff enough to cope with my max power. They all handle without real faults, and although I can detect geometry differences, it's not really important - you can ride around it. I have aero, weenie and "regular" bikes; again, I can tell the difference, and I'd take my R5Ca to the mountains rather than my S5, but I suspect I'd do OK on the S5, frankly, if it had the R5s 34x32 bottom gear rather than 36x25; it's only 3kg heavier (which means 9w at 7%).
On the other hand, the old bikes...
My ALAN Record Carbonio is probably the best example. I've built it cost-no-object with NOS or as-new C-Record, but it's horrible in comparison to any of the modern stuff. It has the torsional rigidity of wet pasta - I daren't really hoof it, but even at 600w I'm getting something like a 7.5% deflection in the BB. The fork is so flexible that it develops near-terminal speed-wobble if you try to ride it no-hands downhill, and you can watch the front wheel moving backwards and forwards as you ride along. The Turbo saddle is period-correct, but profoundly uncomfortable compared to any modern design with a perineal channel. The C-Record aero pedals and toeclips are a faff to use and hurt my feet when going hard (and weigh a ton). The C-Record groupset was described even at the time as beautiful to look at, and so well made that it would shift as badly in 50 years as it did when new. Which I can confirm is true, even when you don't try to use the (completely hopeless) Syncro II. Oddly, the Deltas - which had a poor reputation even when new, to the extent that the pros *didn't* use them, preferring rebadged Super Record "Cobaltos" - actually work quite well.
Every component on that bike (except, perhaps, the brakes) was used by the pros. Some of them even won races. But it's clear that the equipment was a limiting factor. Now I suspect any bike costing more than £1000 new is good enough to win a crit under the right rider, and is easily stiff, and light, enough for anyone on here. Of course we can always go lighter, and the nice thing is that weight is an absolute and easy to measure, but it won't (hush!) necessarily actually make us go faster.
It's like cars. Car reviews used to mean something, because some cars were rubbish. Now they all have 5-year warranties, ECUs, ABS and more power than anyone really needs, so the reviews have to nitpick. Same for bikes. Apparently the new S3 is 9% stiffer through the BB shell. I can't make my old one, or my S5, deflect even at max output (about 1200w, I'm no Chris Hoy), so how will another 9% help me?
We buy new bikes, and geek out over bike components, because it's a hobby and we enjoy it. For pros, it's their job, so unsurprisingly apart from the components that matter for comfort they take what their sponsors give them. They used not to - including having frames built and rebadged, and buying their own wheels - but now every manufacturer's stuff is good enough for them; and for us.