Should really temper enthusiasm here. I've checked Bills message boards to try to get a feel for it and have found pretty much what you've described. They watched him for three years, like you did, and on average don't appear sad he's gone. I can't think of any actual current "elite NFL WR" out there where that would be possible.
To your point, maybe it's a good trade for both teams. New beginnings and all. I've seen some compare it to the Marshall Faulk trade, but Faulk was absolutely great before he came to the Rams. He just got otherworldly after that is all. Watkins comes to the Rams as good, when healthy. If he stays healthy, maybe he'll go from good to really good or perhaps even great. And if that were to happen, I think everybody will be more than happy about the trade.