Some of us have been through many eras of F1, so it's nothing new to see the old guard replaced. We've had several 'golden ages' were we've lost several top drivers in short order, the most obvious probably being 1982 (losing Villeneuve and Arnoux) and 1993-1994 (losing Prost, Mansell and Senna).
I thought Hamilton would be off this year if he won the title, but unless he does a Rosberg (not going to happen!) then he'll be at the top for a few years yet - Schumacher's 7 titles could well be matched. I felt over last winter that his attention had moved and he seemed less focussed on F1, but it seems he's just learning to stay fresh by having other passions. Once he leaves though that's it; he's not interested in other forms of motorsport, which is a shame as Hamilton vs Alonso in similar cars would be a mouthwatering prospect in an endurance championship.
Alonso could also stay in F1 as long as he wants, but it largely depends on what happens in the expected driver shuffle next year and whether McLaren look much better with the Renault engine. Unlike Hamilton Alonso is clearly interested in other forms of motorsport, and at some point he'll undoubtedly move to Indycar or Le Mans Series (depending on what happens there soon, as it's a bit up in the air at the moment).
As for Vettel, I agree with MissChief, he might extend his Ferrari contract for a year or 2, but once he leaves Ferrari he'll leave F1 and possibly motorsport. He looks under immense pressure at Ferrari, so I think he'll quit relatively early for the sake of his own sanity! He strikes me as quite the family man and doesn't yearn for the limelight, so he might do a bit of DTM or something if the juices are still flowing, but I can't see him jetting around the world or Europe for half the year.
Kimi, while probably better than most, obviously isn't what he used to be, but he's lost interest before and come back, so who knows. I feel he could still do a job for a midfield team, if he's interested, but teams are looking at younger drivers now and his time in F1 will probably be over if Leclerc has a good season next year (assuming he does end up at Sauber).
Verstappen is the obvious candidate to take over as the boss, ironically, but there are plenty of other exciting youngsters who could be right at the top. Ocon, Leclerc, Sainz, Vandoorne and possibly even Giovanazzi and Wehrlein (obviously Mercedes still have hopes for him considering they've continued to support him, but his time is likely over) are all drivers supported by top manufacturers, so they have the potential to be title challengers.
