I think it’s sorta okay that the enemies don’t get too much stronger, especially since (at least for casters) a lot of the added power comes in the form of gaining access to stupidly OP spells like Hypnotic Pattern. I don’t think it would be very fun if enemies started using tactics that amounted to “Hahaha, I rolled higher initiative so now you don’t get to play for the next three rounds while I can do whatever I want.”