The thing is, armor is costly to raise, not impossible to raise. Now, with 2d10, you've made high armor worth 5x as much as it is under d20 - now the marginal benefit of raising your AC increases. At the same time, the marginal benefit of raising saves drops, making this an more complex picture. And we're just looking at the value of a single hit so far; when we look at how it changes the graph of the value over an encounter, it changes the scenery even more.
It is both costly and impossible. Costly because of the high price involved, impossible because the amounts needed to get anywhere with it are unobtainable. You'd have to have around double wealth, AND devote it all just to physical defense in order to get a physical defense that means anything. And of course that says nothing for special defense, offense, support/utility...
And this is just flat out wrong on so many levels.
It's exactly right. The lowest possible result that has any chance of success is a 2 on a 1d20 and a 3 on a 2d10. So when fighting something with an attack bonus of x, instead of needing a defense of at least x + 3 to do anything at all, you need a defense of at least x + 4 to do anything at all. Otherwise, all attacks that could hit will. You might as well have defense 0 for all the good it's doing.
And that's the same sort of effect as boosting attack bonuses by 1.
What's more...
Enemies hit on any non auto fail: 95% with 1d20, 99% with 2d10.
Enemies hit on a 3/4: 90% with 1d20, 97% with 2d10.
Enemies hit on a 4/5: 85% with 1d20, 93% with 2d10.
Enemies hit on a 5/6: 80% with 1d20, 90% with 2d10.
Enemies hit on a 6/7: 75% with 1d20, 85% with 2d10.
Enemies hit on a 7/8: 70% with 1d20, 79% with 2d10.
Enemies hit on a 8/9: 65% with 1d20, 72% with 2d10.
Enemies hit on a 9/10: 60% with 1d20, 64% with 2d10.
Enemies hit on a 10/11: 55% either way (the break even point).
So just to even begin getting ahead, you'd have to have defenses of attack +12 or better. Except that that isn't possible for PCs to do for physical defense, so instead they get hit more often and on the special side enemies benefit from that more than the party does.
Someone also pointed out that enemies CAN get high physical defense simply by stacking buffs. Any level 20 enemy could casually throw out a 50-60 AC. For the weak builds at least, that puts them into the same sort of doesn't hit often, now hits even less type situation. Which just means that the party is screwed on both sides.