The original poster is absolutely right. Look at Mike Wallace getting moved today. Wallace was the gem of free agency two summers ago, and Miami couldn't get rid of him fast enough two years later.
Football is such a team sport that the success of even seemingly can't-miss players doesn't always translate to their new destination. It's why, if I'm an Eagle fan, I have some concern about the Murray signing. It took a combination of a lot of things to produce the rushing output he had last year: a game plan committed to the run, Murray's health, Murray's talent, an unmatched offensive line, and one of the best supporting casts of offensive talent in the league. Chip away any of those things and it can produce a wildly different result -- both for the Cowboys and for Murray individually.