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.
In this case systemd seems to be reinventing process groups, in a totally different way, instead of fixing whatever the reason is why GUI sessions don't use session leaders.
So it's pretty obvious there really is a problem that needs to be fixed, and apparently so far nobody else has made a real or successful attempt to do so.