He prolly doesn't want to give his seat away to some German who wants to return to F1. 
Do you think Sebastian Vettel would like a seat at Aston Martin even if he decides to return? I'm not sure about that after having really bad memories with the team.
He wouldn't like to be the teammate of Stroll in the first place. 
Stroll isn't the worst driver on the grid maybe but he still needs to put a lot more into his effort.If Vettel makes such decision I would expect him to eye the Mercedes seat which will be empty after Hamilton joins Ferrari. I guess the German team would lean towards such deal also.
However I don't think he will return as a driver anymore.
I think when someone doing a certain job for many years retires from it and find himself enjoying life for a certain amount of time,carefree I mean the desire to return to do what he used to in life,driving a F1 car will only increase in him and seeing an empty spot at Mercedes can be a big motivation to come back,sure for the last 2 seasons Mercedes has gone ashtray and is not the team we used to see in performance the past years where they completely dominated the sport yet it still has the possibility to start already working for next year car as up until now things do not look good and maybe they convince Vettel to drive for them,a thing I believe all of us F1 fans want to see it happen.
Because Aston Martin doesn't have too much to offer for Alonso at the moment. The car isn't good enough no matter how hard he is trying to get the best result anyway. I wonder if they guaranteed him something for the future.
