I don't understand why people talk so much about Sebastian Vettel's name as a possible replacement for Hamilton.
I honestly can't believe that Hammilton will leave F1 despite the problems that occurred last season, but if that is the case, my bets on a replacement go to Pierre Gasly, one of the highlights of Formula 1 in recent years and, whenever he can, reveals dissatisfaction with Red Bull for leaving him at AlphaTauri, or Esteban Ocon, who has had a connection with Mercedes since the middle of the last decade.
That is mainly because most of the people who got into this sport are around my age or older, sure there are newer and younger generations these days but the 30+ or even 40+ year olds that watch it are a big demographic of it. Which means they have seen Schumacher race, and that was with Ferrari mostly.
So, when Vettel got that many championships with Red Bull and switched to Ferrari later on, they saw him as the next coming, and that is why they loved him, plus he was the first one to actually test Hamilton as well, with a car that is nearly 0.5+ seconds faster, Hamilton managed to get the title but Vettel made it closer than it should be.
Hence, we all love Vettel and if Hamilton leaves, then seeing Vettel in the best possible car would be lovely thing to do. Combine that with the fact that he is driving a car that is beneath his talents, everyone wants to see him race in a great car before he retires.