Donald is a great player.
But he also clearly wants the last nickel on the table for his career. I'm not judging him; I'm saying that this is his goal. The Rams are not known as cheap. But I think it is VERY likely that Donald's agent wants to break the mold for a player's contract and give him provisions that have no precedent (such as giving him huge dollars plus an opt out, or perhaps paying him some amount over the highest defensive player contract and readjusted every year to account for it).
I have said for long time that as great as he is, he is NOT what makes this D work or not work. He is comparable to JJ Watts. As Houston has found out; the D can survive the loss of Watts but what they can't survive is the loss of is a great QB.
Donald is worth paying a big salary to. But not the kind of money that inhibits the Rams from keeping other players that serve vital roles or by setting a contract precedent that makes dealing with other players difficult. The Pats figured that out some time ago understanding that there's perhaps 2 maybe 3 players on a team worth breaking the bank for. Only 1 worth arranging an entire team around. Otherwise, if you pay too much to too few, you can't keep the other positions well stocked.
If the Rams know that they cannot keep him long term without destroying their salary structure, then it boils down to a couple of choices, both involving hard ball.
1) Trade him this coming year when he is at peak value. I don't know what he could bring, but I would think it is probably a good starting player plus a 1st round pick, or a top level player plus a 2nd round pick.
or 2) Leave the last best offer on the table. If he refuses (which he apparently has), then hold him to his contract for 2018. Franchise him for 2019 and 2020. And then lose him in 2021. We'll have received several years of his peak service but at an affordable cost. That's life. Football is a 53 person team; not 1. And even if a QB like Tom Brady is a guy you'd lose almost everybody else to keep, that is NEVER the case with a DT. They can only affect a game to a degree. Nowhere near the degree that a QB, or possibly a great CB, can.
Let me say again; I'm a Donald fan and he deserves to be paid as perhaps the top DT in the league. But he is NOT the best defensive player nor is he the most influential defensive player. I doubt there is such a thing.