There is a good saying in Italy. Stucco e pittura fan bella figura ( you can slap some mortar to fix minor holes and some paint to look new and fresh, but underneath it's still same old house with all structural problems). In the context of D&D, stucco or mortar is minor mechanical tweaks and updates, pittura is new book design, layouts, artwork. But underneath it's still 10 year old game chassis. There is a limit how much you can tweak without needing to overhaul entire system ( and then it becomes new edition) and without breaking more or less plug and play compatibility with 5ed adventure books and supplements.
Same thing happened with 3.5. You could more or less use all the 3.0 splat books with minor tweaking and sometimes without any tweaking at all.
There is a limit how much you can add, remove, tweak around and generally improve in increments (when we talk about mechanics). But sooner or later, you reach the limit where you can't do much more and you ether do new edition or you do overhaul so drastic that it just may be new edition in all but name.