I agree. If Keenum is the better chance to win in the short run (a few games) then he should play, period. Which is again different than some teams that took a QB #1 overall and may have been 2-14 or 1-15 would approach it.
Personally I'd probably play Keenum 1/3 and Goff 2/3 this Saturday @DEN and then Goff and Mannion 50/50 in the final game @MIN.
Another possibility that could play out well for the Rams is if Keenum plays pretty well- for however long- and he attracts a signing elsewhere in the spring it could yield the Rams a 2018 comp. pick. Not a big one like a 3rd or 4th rounder of course. But maybe a 6th. Who knows. Just saying it's possible.