Alright, I wasn't planning to join the fray on this particular discussion, but it seems necessary given the numbers that are being thrown around.
Yitzi. You can't adjust the rules for PCs while completely ignoring the rules for monsters. It doesn't work that way. If you lower the power of every class while letting monsters stay just as powerful, the CR system falls completely out of whack.
AC is a broken mechanic. This isn't something you can argue, it's fact. Unless you can wear full plate, and more than half the classes can't, your AC will be 30 or less unless you're shapeshifting somehow. (10+10 for a +5 breastplate +3 Dex is 26 for a barbarian, 10+5 Class +10 Wis +5 Dex is 30 for a tank-monk, 10+9 for a +5 chain shirt +10 Dex is 29 for a rogue)
This means, if you're getting full-attacked by a monster, you're going to get hit almost every time at higher levels. So how do you kill a monster? You have to nerf it somehow before you charge in.
It's like...*shudders* It's like Final Fantasy XIII. In order to make every role seem important, rather than just blasters and DPR, monsters gain all kinds of immunities to melee and energy damage, so you have to stun or daze or petrify them instead of just straight up hitting them with your sword.
You, on the other hand, have nerfed DCs and debuffs, as well as PC melee stats, while keeping all monsters untouched.
Now you're going to charge at a monster who was CR'ed with the intent of facing PCs capable of casting miracle, flesh to stone, and destruction, but with a 29 in two stats and your defensive spells killed, no Shock Trooper, no Heedless Charge, etc.
You know why non-Core exists? Because the MM is poorly designed. Because regeneration, DR, level-draining, rending, grappling, swallowing hole, and other things, are poorly designed. Because you're going to fight monsters as superhumans, when you need to be a monster to fight a monster.
If you want to kill the tarrasque with damage, you're doing it wrong. If you want to kill a dragon with damage, you're doing it wrong. Monsters are designed to be challenging to sub-optimized, equal to optimized, and devastating to unoptimized, at least in Core-only (which is why all that extra material was released. Because monsters are too powerful)
So by nerfing damage, by nerfing debuffs, by nerfing stats themselves without improving AC or weakening enemies, you're basically setting up higher level games to be completely unbalanced and challenging, requiring immense amounts of optimization from the players who can't optimize because there are limits on their power, when there are no such limits on monsters.