I agree, please no. I mean, on one hand I like the idea of Supergirl existing in the same universe as the Flash. But, on the other hand, the showrunners have already established they exist in two separate universes. If there is an Earth-Prime Supergirl, it'll only create confusion among the fanbase.
So, I really, really, really hope the showrunners don't play with this idea of an Earth-Prime Supergirl. We don't need it!
Agreed, Earth 1 needs Kryptonians about as much as I need a hole drilled though my head with a power drill and no anesthetic.