The fact is though that when he won last Championship he said a clear message to the Redbull team let's do this for another 10 years to come so I think it will be difficult for him to move from Redbull if Redbull keeps offering him a winning car.
I don't see a single reason for Verstappen to switch at the moment, Mercedes already have their Hamilton successor, McLaren haven't had a car that could dominate for years and Ferrari is a separate issue, remember Kimi was the last world champion there, all the other big ones including double and quadruple world champions have failed miserably at Ferrari, why would Verstappen even begin to do that to himself?
Now that Porsche is also joining Red Bull, you have a traditional name on the car again. Maybe every great driver wants to become world champion with Ferrari, but only a few have really made it, which might also be a reason why Hamilton never switched to Ferrari.