I think OS4/MOS in particular need each other more than most users care to admit.
Sure, you get fanbois on either side that like to slate the opposition at every opportunity, but they are, without exception, idiots, in my book.
As long as both OS4 and MOS exist, there is competition and with that progression overall. If either one died, it is very unlikely it's user base would migrate over (hell would probably have to freeze first) and you'd be left with one expensive proprietary OS on fundamentally obsolete hardware that, in the absence of it's old sparring partner as a metric, has absolutely nothing to offer over free alternatives on up-to-date hardware. Only the most dedicated users would remain.