FLOSS will simply evolve to leave the systemd-limited distros behind.
I think that, given time, the problem will take care of itself since nobody will be able to come up with anything that is both enough of a significant improvement on System V init to get System V init anywhere near what systemd does even today, compatible with tradition enough so the die-hard System V init fans won't complain, and gets traction in enough distributions so people will actually be interested. (The ones which have subscribed to systemd already aren't going back unless whatever the System V init camp has to propose is really a lot better than systemd, which would be quite surprising.) This is very unlikely to actually happen since historically the distributions didn't even seem to be able to agree on a standard for init scripts, let alone all of System V init or indeed an evolutionarily improved System V init.
In the meantime, systemd will improve even further and, with the major Linux distributions behind it, will become even more compelling. In effect, System V init will leave systemd behind in exactly the way that CVS left Git behind.
Feel free to prove me wrong.
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