I don't think it necessarily means Fournette is done. They only gave up a fifth round pick for Hyde. The Dolphins got a fourth round pick for Ajayi last year when they traded him, and Gase wanted him off the team no matter what. The Jags are 3-3 and Yeldon is iffy (or possibly worse) for this week. If they lose this week and next they might not make the playoffs.
Even if Hyde is the starter for only 2 weeks, if they get a win both weeks, the trade is probably worth it. And Hyde would still be a valuable backup at a somewhat reduced price, since the Browns are on the hook for his signing bonus against the cap, so you're not looking at a massive cap hit against the Jags.
Of course, if Fournette is healthy after the Bye, having Hyde should mean he gets spelled more often than he would be without Hyde, so this does lower Fournette's ceiling. And it's possible Fournette really is done, although as a Fournette owner who has neither Yeldon nor Hyde, I hope not.