I can see, for several reasons (winning team, friends like Tatis on the team, weather, etc), SD getting a little more benefit than us, but I still think its 99.9% chance he goes to FA. As others have said, that was his plan all along. The thing going against SD is their payroll already is insane, so they won't be able to afford his contract. Guessing he's just a very expensive rental for them for 3 playoff runs.
I think the Nats are going to have just as good of a chance signing him in FA in 2.5 years as they would've if we didn't trade him.