-snip-
If they look convincing this season then who knows maybe they will even convince Verstappen to sign for them as well.

It isn't mission impossible in theory. Like I said before, he doesn't have a special bond with Red Bull. He just wants to be a winner.
If they can't offer him better standards at one point, he would seriously think of leaving. No matter how much he likes to work with Helmut Marko. Fernando Alonso won't race forever. Aston Martin would probably bring Verstappen in his place in that case.
All of that could happen, for example Hamilton himself is able to leave Mercedes, the team that made his name and made him win the world title 7 times, let alone Max, he can move anytime if he wants. But when it will happen no one knows, what people can do so far is only speculate about his transfer to another team, but until now there are no signs that he will leave Red Bull. So now Aston Martin or any team that expects Max can only guess, the decision remains with the Dutch driver.