Perhaps the thinking was something like this:
The Rams are good enough now to win with Keenum (based on the defense in general and the Dline in particular, Gurley & the rest of the RBs, and the improving OL). Next year they'll be even better with Goff at QB ... and Goff will improve the team more than all the guys they could have drafted would have. Plus if it works out he'll do it for a decade or more.
However, if it turns out that there is a talent deficit because of the draft picks the Rams traded they'll be in position to sign a couple big-money FAs to fill the gaps because the QB position will be so cheap (by NFL standards) with Goff and Mannion both on their rookie contracts.
AlbaNY_Ram