The kind of numbers and thoughts you have on Watkins aren't far from mine.
He might be able to be a more prolific receiver in a different offense on a different team. And I have little doubt that an offseason with Goff and a full training camp and we'll see more attempts thrown his direction.
But the Rams offense simply is not going to feature the 1200 yard WR with the other guys getting the leftovers. It's going to be spread around. And we have to remember how much McVay leans towards TEs. And last year the TEs were only marginally involved in the passing game (you can bet all that can be done to remedy that is going to happen in FA or the draft). So if any position sees more passes going their way next season, it's going to be the TE unit. That means even FEWER attempts to the WRs or MORE passing and less running (doesn't sound like McVay).
Either Watkins wraps his head around the concept that he is NOT going to be the big Cahuna on the Rams; he's NOT going to get a Dez Bryant, Julio Jones type deal or get as many passes thrown his way as those kinds of guys do; but rather he is a complementary piece. If he can't live with that, and get paid like that (a Robert Woods sized contract), then it is best to just let him walk, eat the 2nd round pick loss, and move on rather than hamstringing the team by spending too much money on a position that doesn't warrant it. Especially when there are other players we need to retain, and a couple of holes we need to fill.