I can't think of any good reason to restrict push to a smaller group than the current set of svn committers. New or occasional contributors can still submit patches via Mantis, to be given push when they've earned the trust of the project
admins, just like they do now. That's pretty much how all of the larger projects (Gnome, Freedesktop, Cairo, Apache) handle it.
Linux is the exception because they didn't use a central repo until recently; Torvalds wrote git specifically to avoid needing one
and designed the workflow accordingly.