How does this matter? Foles is not the starter. So your gonna give up draft picks for a backup likely to be cut? It makes no sense. Keenum is going nowhere. Mannion is going nowhere. So the only logical move is Foles for McCown straight up. If we draft a rookie, that player is cut anyways.
Yes, we're trying to upgrade the starter, not the second or third string QB. Certainly we wouldn't want to give up ANY resources to improve that position at this stage.
Foles will be kept around for part of training camp, because injuries do happen, and they will want 4 QBs going into the start of camp. The only way he's not around for then is if some team trades something of value for him, even if the value is fairly small. The Rams are short on draft picks, after all, and may wish to move up.