Simple isn't so bad in and of itself, but 5E is not only simple, a lot of things that there should be rules for, there's just "whatever the DM says".
Mmm... anything specific in your mind that really
should be covered by 5e rules but isn't? I cannot think of anything
essential for a TTRPG in the general case. The rules do cover practically anything, they just don't differentiate the details, so the "DM rulings" are in most cases just the DM calling for an ability check and setting a DC on the fly, maybe granting (dis)advantage with it.
But we're talking about a CRPG here, and there's no reason why the game designers can't do the same i.e. allow to use abilities on certain elements of the environment with a DC under the hood.
If you are concerned that you won't be able to do something unusual or particularly creative, good luck finding
any CRPG that allows you total freedom, it just doesn't exist.
It is, however, a totally different genre of play than action games, which people happen to enjoy for it's own merits, such as accessibility and the emphasis on strategy. It is not a downgrade or more primitive combat system for a game.
Absolutely! Some people like FPS or shoot'em ups and just don't get how is it possible that other people like turn-based, it's their problem. And just because I do like turn-based it doesn't mean I hate those in real-time, I just think we have plenty of them while we have few precious good turn-based games (BTW I never liked early Final Fantasy games TBH because of the total lack of movement during combat). I want an option for a game that requires MORE thinking and LESS reaction speed, one more real-time game adds nothing to my gaming options.
Turn based combat takes a lot longer than pausable real time and that negatively impacts pacing.
People playing PnP RPGs complain all the time that combat takes too long. Guess what, its no different on a PC.
The complaint in D&D is about combat taking too long
compared to the rest of the game, which is supposed to also have a narrative, social interactions, exploration of locales, equipment management (possibly including shopping if you like) and more.
There are some computer games where combat is very fast, and
it goes on all the damn time (Diablo III). Oh wait that's no true, not all the time, because every 10 minutes you have to stop and empty your equipment from all the useless treasure by going through the recycling mill...
If your primary interest is in a high-paced game, Diablo III might work for you, but I wonder are you sure you like D&D? Because D&D has
always been a slow-pace game after all... For my personal tastes low-pace is preferable. It doesn't mean I want each turn in a RPG to take an hour, it means I want to have
time to think. I can't have that in Diablo III (I still play it but I really consider that game a shoot'em up), but I also can't have it in WoW for example, I can
prepare for a battle, but then it's all about reacting quickly and mostly going through the motion of an established tactic.