@eniko i do not really know, i've written games from scratch, but never really developed a "product" (from scratch), i'll just share my side of the coin:
part of my regular day to day is fighting with the engine (and external development software), developing small tools and shortcuts often with lacking api-s and no documentation and "solving" weird bugs in serialization, asset handling, display events etc. by doing weird workarounds
an open-source engine might allow you to mitigate all of those things, but that's like saying being able to read the code fixes it, it's still a ton of work however much easier it is
so I guess no, that's probably a bad idea with delayed consequences?