Not that we should rely on processes to behave nicely, but that we shouldn't break processes which are because of some which aren't.

In particular, if Gnome is starting a ton of background processes that stick around and cause problems, then the fix should be to change Gnome, not to change the default behaviour and break everyone else.

To use the OS sandbox example, this is the reason why OS X added application sandboxing and forced Mac App Store apps to use it, and not anyone else. Systemd's approach is like Apple saying "We're going to start sandboxing everyone's apps, App Store or no, unless you make these changes to your apps.