the value he provides is that he is a dynamic playmaker that is still plenty young. he is fun to watch - something this team is going to need, frankly, and he is going to make the offense, and a lot of the players around him, better. whomever is our QB will benefit from him. he is still on his rookie deal. rebuilding doesn't need to take a decade in today's NFL, and it requires you have some continuity in players during that rebuild, usually. his contract isn't an issue right now and if we trade away every incredible talent we have to avoid paying them money, we'll never actually finish our rebuild.
obviously if some basement dweller offers us an incredible haul you consider it - as you would for anyone - but realistically, nobody is going to give us more than a late 1st or a couple 2nds and he is more valuable to this team than those picks would be over the next 3-5 years, and his contract could be structured to give the team some freedom to build later if they get the extension done soon. plus, RB payscale isn't as punishing to locking up a player as, e.g. QB and WR payscales are.
I do not understand any argument save one that is an emotional "burn every single thing down" to trading CMC this year.