The thing with Vettel is Ferrari are paying him a huge amount of cash to be the leader, the champion who will steer them back to glory. Except he's being shown up by his second year teammate who is hungry and very fast.
If Leclerc can keep evolving at this rate, he'll go into 2020 wanting to lead the team. And they'll have faith in the kid to do just that. He's that talented and intelligent, which is why they promoted him so quickly.
That being the case, where does it leave Seb? Sure he could stay (like say Piquet in his Benetton years) but he'd be on peanuts because they could replace him otherwise. His performances haven't justified his salary for a long while...
Would he want that? I doubt it. Yet I don't see many other options within F1 that would appeal to him.
Of course this is based off him not turning things around fast. You never know I guess.
I think Nelson did well out of that deal. Maybe that is Seb's plan, get an incentive based deal based on low expectations, and then win everything.