Additively regarding proprietary vendor's changing business model, forking of parts of the stack by FOSSees is impactful in the same way. Yet there doesn't seem to be a high bar limiting forking like there is for a changing business model.
FOSS works a bit like evolution, survival of the fittest applies. If something is forked and the fork wins over the original, there is probably a good reason for it (ie. the original developers had a flawed approach that was untenable). That's one of the great things actually, with FOSS you can challenge the original developers and make improvements that they might not agree with.
There is actually a very influential book by Eric Raymond called the "Cathedral and the Bazaar" that goes into much greater detail on this sort of thing, better than I can.