Will Sebastien be retired in 2021 due to lack of a ride?

I can see him at RP (or AM).

I think any open Alfa Romeo seats will go to an F2 driver. Ilott or Shwartzman...maybe Schumacher (I'm not sure he's the best choice among the three just yet).

 

Well, the puzzle pieces need to come together.

And, after today: https://getyarn.io/yarn-clip/7289a890-5296-4979-91b2-b2361f5b1bb9

 

I can see him leaving F1 if not at Pink Clone. The only reason he goes there is Mercedes PU as thats as close as he will ever get to being in a real Mercedes. His 22 car will not be a clone and might disappoint as I expect Red Bull, Ferrari and actual Mercedes to show up with competitive vehicle.

Also - Unless the car has superior pace advantage, and a soundly planted rear end, Vettel is not the best out there at all. Not close. He lacks the ability to drive around issues and like Kimi needs a suited car from the off.

 

It's very much the "last chance saloon" for Vettel if he is recruited by Aston Martin.
Vettel apart, this year's Pink Clone, as you call it, may simply be a stop gap solution, necessarilly carried over to 2021, as per FIA rules.
But I don't think Lawrence Stroll bought Force India and rebrands it Aston Martin next year only to be a servile Mercedes customer and forever copy their cars.
I expect the technical team at Aston Martin to be seriously beefed up and in future create their own designs like McLaren does.

 

Crystal ball says Vettel will be on the grid next year