https://en.wikipedia.org/wiki/John_Surtees
Wouldn't it be cool to see someone make that move again? I think Mike Hailwood was the last to give it a shot in F1 and I'm pretty sure they only gave him one season. Jeff Ward went from MX to Indycars and did well given his equipment.
Acadian's right that Vale would have had to jump earlier, but I think his tests showed he could have been competitive. Lorenzo had a good test with Williams earlier this year. If someone is actually going to try, though, I bet it would be Marquez. By the time he hits his mid-20s, there won't be many records left to break and nothing left to prove. Maybe he starts looking for a new challenge.
Back to Mercedes: The "we're in no hurry" stance tells me they're hoping to pry someone away from another team. Probably won't be Vettel -- you can't buy something from Ferrari unless they want to sell because they don't need the money. I can't see Red Bull letting either driver go, or even letting Sainz out of his contract, but they have a logjam in their junior program, so maybe. McLaren is a billion dollar business but they haven't had a title sponsor in years, so you have to figure the F1 team bleeds money like a severed artery. Maybe they'll sell Alonso. Perez and Wehrlein can be snagged any time since Merc supplies those teams with engines so expect one of them to be the fall back.
I think they should put Wehrlein there for a year in a trial by fire. If he doesn't do well, Alonso and Vettel are free after next season. If he does do well, they'll save a buttload of dough on their 2018 driver contracts. And either way, it helps them recruit to their junior program because they have an example of someone who graduated to the grown up team.
But, shockingly, the bastards haven't asked for my advice.