Then the primary issue is the rebalance of monsters, aka high attack number low damage vs single attack high damage monster.It's true...
Though, full disclosure: I'm thinking of doing this for -players- only. With NPCs retaining a singular AC and no additional DR.
though that said, frankly I think the death save rules of 5e already make more attack monsters generally deadlier than single attack monsters (once PCs are 5thish level).
So pushing the needle bsck a bit might be a good thing