Yep! That's what makes the Patriots so smart and sustains their dynasty (of course, with a little help from Brady). They know when to move on from players. So they'll ship them before they hit the end of their contracts, and with the draft pick they acquire for trading the player, they retool.
The problem with the Cowboys, in my opinion, is that they have a "one player away" mentality. So they keep almost all of their players until they reach the end of their contracts because they think if they acquire just one more player, everything else is in place for a Super Bowl trip. If the Cowboys knew by 2018 they weren't going to keep Byron Jones, they should have shipped him off for a second-round pick, at the very least. Don't way until his contract is done. Move him out and find his replacement in the 2019 draft.