Yeah...well two things.
First, the team holds all the cards. Hopkins would need to hold out and forego a large amount of money; in addition, the new CBA does not allow teams to rescind holdout fines, so that's a lot of money Hopkins would be unable to recover.
Second, it isn't so much that they traded him but the return they got back. A second round pick and David Johnson...lmfao! Just look at the trade market for these guys...OBJ fetched a first and third. The worst possible value was established when they then used a second round pick and traded that for Brandin Cooks...lmfaoooo! I mean, you're swapping draft comp to trade a top 10 NFL player and arguably the best receiver in the NFL for Cooks. How can that happen?
On the other hand, they traded two first round picks (!) for Laremy Tunsil, a guy who is good but not even elite and not all-pro level, and who doesn't even appear to be shoring up their blocking. And they paid him by far the most money of any LT in the league, yet they wouldn't use that to extend Hopkins. This was after they traded Duane Brown, their longtime excellent LT, for a sack of potatoes because they didn't want to increase his salary (his demands were very reasonable.)