I would think that critical core/security updates would be possible to maintain for as long as legacy RH 7.3 updates are available, but developers resources are limited and really focused on new coming SME 7. Start testing version 7 and preparing for migration now or wait for the "official" release and test then. Even if SME 6.x doesn't have known major security issues now, it will not be receiving as much attention from the developers as SME 7 in a future.