If I'm not wrong doesn't giving easy advantage reinforce the problem even more than reducing the AC by (let's say) 1 or 2 points ? On the other hand I noticed that they increased the dexterity of many creatures in the game compared to the MM, which probably increase the issue you raised.
Serious question : In the PnP, aren't the spells that targets AC used less often at higher level when characters have higher level and more powerfull spells that require a ST ? It's something Saito brought in another thread and I found interresting to think about (as the non PnP player I am).
Both advantage and AC adjusting are contributors to that problem.
Spells/attacks which target saving throws might be more common at higher levels, but not all saving throws are equal. Lots of monsters have high CON saving throws, but not a lot have high INT or CHA saving throws.