With Hamilton to Ferrari, I'm a bit on the fence (for 2021 that is).
When Schumacher went to Ferrari, he was part of the core team of Benetton that moved over to build, together with PM who were leaving McLaren. It was a big project with the aim to change formula one, which they did.
Hamilton isn't part of this kind of small team that build Mercedes like Benetton had in the early nineties, Mercedes (and Mclaren when they were part of Daimler) is more about structure then specific people. Verstappen in a few years, together with Horner and his team would make more sense.
Liberty and big sponsors might have a bigger problem in 2021: Kimi probably retires at the end of 2020 and if Hamilton takes 2019 and 2020, there will only be Hamilton and Vettel as world champions. For marketing Hamilton (and Vettel to a lesser degree) have to stay on until Verstappen and/or Leclerc become world champion. Just like Alonso took over from Schumacher. last time F1 was without world champion was in 1994 and they lured Mansell back into the sport to do some races.
So, I think Philip Morris will keep Vettel next to Leclerc for 2021 and 2022 (as long as they extend their contract with Ferrari) and the same for Hamilton at Mercedes where they will move mountains to get Verstappen for 2021 (if they aren't doing that already, a corporation like Daimler won't wait until next year to find out what they are going to do)
Other possibility of course, with the Concorde agreement ending, RedBull might leave and sell their team to PM who are out of contract with Ferrari and we'll have Marlboro Honda again... Ferrari would go back to the years of Alesi and Berger with Leclerc and Bottas, etc etc etc...