+1 to the idea that we really do not know how many programs will be affected by this change. We do know that the tmux folks have shot down making any changes to accommodate systemd. As they value cross-platform compatibility, this is understandable.
As it looks from my vantage point, the choice is either carry a patch to revert this change in systemd, or accept the load of carrying an unknown number of patches to allow other software to accommodate this change.
My suggestion is to plan on reverting the systemd change to KillUserProcess=no in F25, and reevaluate for F26, when we have a better understanding of just how many programs have to be rewritten/patched/forked to accommodate this new paradigm.