Lewis going to Ferrari is both imagination and fascination, but not practical. Regardless of what Lewis talks, he is a very practical man and he knows the the possibilities of contending for WDC with Mercedes are higher due to the strength of the team. He has established himself well and the team works around him well with clearly established working mechanisms where they understand each others' needs very well.
Ferrari is highly volatile when it comes to team and car performance. They have a very different work culture to UK based teams and any new driver would have to work hard to get himself positioned in the team the way Lewis is currently in Mercedes. It's not a short term exercise and Lewis has made it clear that, he doesn't have it in him anymore to repeat it.
He is not going to retire either. He knows very well that, the more championships he wins, the more it would serve his brand for whatever he does after F1. He is in perfect shape physically and mentally and more importantly, seems to be enjoying his time with his own performances at the peak of his powers. He won't let the opportunity be passed for winning a few more titles. I bet he would see through 2022.
Ferrari has found what they need, Leclerc. A young, fast, hungry and aggressive driver who can turn in and perform the way he has done in the past few GPs. He is bound to grow and add a bit more speed, which is what Ferrari wants for future. They have got their star and they only need a great support act.
It makes sense for Mercedes to put Russell in W12 in 2021 and see how he performs. If he performs the way Leclerc has done, Mercedes would have hit the jackpot and would be happy to see Lewis retire at the end of 2022. They can then bring Ocon back for 2023, if he delivers in Renault for the next two seasons.
https://www.grandprix247.com/2019/10/03 ... -mercedes/
With Lando now being managed by Mercedes, they have a number of options to evaluate.