Have a search for OS user market share. You’ll find mixed % depending on context (Steam for example shows 7% still on Windows 7, whereas general usage finds 25-35%), so it all depends on your target audience. My experience seems to differ from yours dev-wise. Most Win 7 game devs I know are small single-man indie devs, though.
The whole point of using another game engine is supposed to be to remove the hassle of having to integrate changelists and fixes, recompile engines and editors, etc.
I would understand your point if maybe
a) Compiling the source was nice and simple (it’s not),
b) Integrating a required fix was nice and simple (it’s not) and
c) It was just obscure weird bugs that exist in UE4,
but it’s not. There are huge, glaring, critical bugs existing in basic core systems (AI, Texture rendering, Landscapes, Importing, etc.)