Mannion is unproven but with this team, spending $6.5M on a Shaun Hill like deal for Keenum would be crazy. Keenum is the worst QB in the NFL and hasn't thrown a TD pass in 14 quarters straight. Some want to sugar coat and gesticulate about why he's good despite what we can all see, but he isn't worth much of anything as a starter and would cost way too much as a backup.
I don't see any value in him knowing the system or having 8 more starts of experience in it since the system sucks. Mannion and Goff will be fine without Keenum around and the Rams can use that cap space to help the team re-sign and sign FA's to move forward without all the draft picks they spent.