I have to agree that I don't like this change. It seems like we are addressing a symptom, and not the underlying problem. You say there should be trade-offs for wearing heavier armor, but you're not introducing any compromises. The current change only forces a new limitation based on class choices that some of us made 5 or more years ago. Introducing caps (to anything) is a poor way to fix the system. It only limits what we can do without allowing choices.
I would rather be allowed to pick and choose what equipment I wear to meet my objectives. If my objective is to hit a certain AC, there should be disadvantages of course but it should be possible. In my opinion, the broken thing about high AC is that you can get very high AC along with +damage, +spelldam, +mana, +regen, etc. Eq with the high AC values should have no other benefits or also come with disadvantages.
If a tank must decide whether to run -11 with 300 extra HPs or with -12, that's an interesting decision. Forcing him to run at -11 takes away his chance to decide for himself. Similarly, a mage should be able to decide whether to run at -9 at full spelldam or at -11 with no spelldam at all. This method uses EQ to allow players some flexibility and choice when they play, rather than forcing them into a role defined by the someone else.
I have said before that designing eq that allows characters to run at their "max" of anything while still attaining the max AC was a silly design decision. Thieves should have to choose whether to run with a wuss stab and good AC, or great stab and lower AC. It would be better to make this a function of equipment worn rather than forcing it based on classo. High end AC should have -hit, -dam, -mr, -mana. Top of the line AC (forges etc) could have high AC without the negatives.