The thing is - Keenum would be the best QB to start the season, and then backup, available to the Rams. So he has real value to the team.
However, he is a free agent after 2016 - and will be worth more to other teams than he is to the Rams, so he will be gone. The Rams will get some comp pick unless they end up signing more free agents, but it is highly unlikely he will be a Ram in 2017. So the question is - can the Rams get more in trade than he is worth to the Rams in 2016 plus the value of a potential comp pick? Oh, and let's not forget that trading him would save the Rams a few million in cap space either to get somebody this year, or to roll over for next year - that is a bonus too. So can the Rams get enough in trade that it and the cap savings is likely to be worth significantly more than Keenum's 2016 as opposed to another QB on the rosters 2016, and the possible comp pick? Add in that if it means Mannion gets more active time, it might increase his value in trade or compensation going forward.
I don't know. I don't know how much playing time Snisher expect the #1 pick to get out of the gate. I don't know what sort of offers are coming in. I especially don't know how they evaluate Keenum vs Foles with an offseason to get over 2015 vs Mannion with another offseason to prepare. I do suspect that it will take a nice trade offer - maybe not pick #63, but Denver's third plus later picks at least. That would allow the Rams to draft a receiver who might be able to contribute, at least.