But that's usually the case, right? It's not easy knowing exactly when to trade someone. Yeah, they could have gotten tons for Gilmore, for example, after his DPOY season, but at that point (after 2019), he still had two years left on his deal and of course the Pats, you know, could use a guy just like that. The reason why a guy would get a massive return is because he's probably a great player but trading away great players is always a huge risk. Yeah, they might get a first round pick for him, but it's entirely possible that the deal still is a net negative when all is said and done.
The odds of trading away a player (1) at his peak value and (2) at a time and in a context that doesn't hurt you, are pretty low. Not that it can't be done, but it's not easy.