If he was killing it, sure. He’s not this season.
Also where would he even go? Stroll’s seat would be an upgrade and that’s it.
I don’t think Sainz is doing poorly or anything, but he’s hardly setting the world on fire either. I’m not sure the leverage is particularly in his favour here, Ferrari have Leclerc already and there’s several drivers of at least the same or similar calibre to Sainz who they could look at as alternatives. So I’m not sure the threat really works. Not saying they’d definitely be looking elsewhere off their own back, just that if that’s his attitude they have no reason not to and they’d find other ways to go. Why rock the boat?