yeah, in the end, as I've stated before, I think MAME will become more of an environment / canvas, where machines can be added / removed on the fly.
at that point everything will change anyway.
external software will probably be able to talk to the running instance, telling it to spawn / despawn a machine as well as that ability obviously being available through the menus.
the existing launch method would likely just be a shortcut for the more complex behavior at that point.
likewise, at that point I imagine front-end authors might want to rethink their approach anyway as you'd potentially have an 'always running' copy of MAME and need to flick between that and your frontend to add/remove machines.
of course we're not there yet, and it could be 10-15 years before that is a reality, if it ever happens, but as MG says, don't expect the project to bend over backwards for frontend authors, that isn't really a consideration.