According to me the real question is "If the dices have been changed twice because players missing attacks might be anti-fun, then why isn't the ennemy's AC reduced?" That's the only goal of ennemie's AC. Lower AC wouldn't have any consequences except that we'll miss less often. They have to know what they want : if missing is the main problem then they could just reduce the AC... Problem solved. Am I wrong ? Is there something I'm not thinking about related to AC ?
Lower AC does have another, major consequence: Lowering AC makes it easier to hit enemies with things which target their AC, but it does not affect things which require enemies to make saving throws to avoid or reduce the damage. The major consequence is that this devalues saving-throw attacks, and makes AC attacks more attractive. This upsets the balance between AC and Saving Throws.