I will say that pure climbers don't exist as much anymore. Most GC riders can usually also ride a decent TT these days. Though there remain exceptions. (Uitdebroecks, widar, Gall)
I would suggest that over time what we consider a GC rider has changed BECAUSE of the ITT-less routes, making some riders GC riders who wouldn’t be before: Carapaz, Yateses, Aru, Landa, Onley, etc. All great riders, but in another era, not really contenders. (It’s similar to how clinic-fueled performances remain embedded in many peoples minds as a form of “truth” when really they are not).
True GC riders could always climb. Sure there were always blips like Moser, but he only won the Giro when the scales were tipped.
So adding more ITT would not change the very top of the hierarchy. Pog would still win, Vingo likely second, tho Remco might come closer to second with a couple of longish flatter TTs. No he would not gain much on Pog, but recently he has put a fair amount of time into Vingo. Beating him in GC, probably not, closer more than likely.
After those three, you would have Rog, Lipo, and then definitely del toro, Ayuso, seixas, Almeida who are all strong at both but perhaps a touch weaker in one.
Again it’s about finding a balance. I don’t expect all fans to agree exactly on how many ITT kms should be in, but suffice it to say that right now there is not enough, while in Indurain or Hinault’s time perhaps there was too much.
In fact, I would suggest Hinault would not have the palmares he has if he competed on today’s routes. He may have been even more climbing-challenged as a GC rider than Remco. His 1978 breakthrough came due to his dominance in ITTs over Zoetemelk who was arguably the better climber.