I'm still not convinced.
Enemy behavior should be more complex than always target the easiest to kill characters or always target the beefiest. There's a wide range of factors to take into account when an enemy chooses to attack a target.
1. Proximity
2. Damage amount dealt by PC's to said enemy
3. Ease of hitting the said PC
4. hatred of archers
5. hatred of mages
6. Proving the enemy is the strongest on team enemy by taking on the hero they perceive as strongest
etc. (and all of this is just with human level of intelligence enemies enemies)
The point is that some enemies should take into account the high effective AC, many others should not for a variety of in game reasons. In your proposed solution, the solution limits the behaviors my monsters can take and makes them less real and more binary. Intelligent ones always ignore the hard to hit character and dumb ones don't. There's a whole world of intelligent enemies that may have a more overpowering motivation than their intelligence. There's also the occasional dumb enemy that may see that the highest AC character is the only one wearing red and attacks him because he wears red or stands out to him in some other way.
In other words, throw variety at your players. Variety of monsters, variety of enemy tactics, variety of battle conditions. At least that's my opinion. So your solution would never work for me. It directly affects my ability to throw variety at the players.