Even when Goff gets elevated to #1, I don't think there is any limiting factor as to why Keenum couldn't be #2 the remainder of this year.
In fact, it would probably be better for Goff to have Keenum #2 because he'll be activated on game day and it would give him someone with NFL game experience to talk things over with.

I do think that after this year, assuming Goff doesn't completely flame out, that Keenum will be gone and our QB corp will be Goff and Mannion.