> What is your point exactly, that developers use new features of APIs/libraries?

Exactly. We use whatever new standard makes our live easier. If Firefox or Edge don't support that standard (because they are always lagging behind), there's not much we can do (except going the hard way and implementing workarounds for each browser, which is a no-no most of the time, because of lack of manpower/time). "My website requires Flex support" is pretty much the equivalent of "My game requires DirectX 11".