i don't think it has anything to do with D+ or over saturation. Including movies, (with a few exceptions) Marvel has been making mediocre-to-bad stuff since Endgame and Star Wars really the entire time Disney has had it.
I guess D+ added more volume of junk, but to me it really is as simple as "just make good stuff". If it's good it's hard to over saturate.
edit: also I am not one of those people who likes to shit on marvel all the time. I loved phases 1-3 and endgame was one of my favorite cinematic experiences ever. I want to like all things marvel, they have just not been firing on all cylinders recently. Pretty much the same with star wars.