From what I have read, userland testing of even RC versions seems to be
as infrequent as to practically be non-existent outside the major
frameworks, having a usable number of people playing about with it when
it's not even in for certain seems unrealistic?
Would it not also lead us to a situation where at any given time this
incubation branch, or god forbid master, could be in any of 2^n
different states depending on which engine features were available or
not? That's a lot of extra uncertainty and possible unexpected interactions.