To take this away from the very dull conversation of decades...
I was thinking recently about the great 2012 stat where we had 6 WDCs on the grid - Hamilton, Vettel, Button, Alonso, Raikkonen and Schumacher. I was trying to think of times when we had no Champions on the grid, and I could think of some races in 1994 after Senna had died and Mansell wasn't on the grid. Have there been any other occasions when this was true (except 1950, obviously)
We could have this situation in 2-3 years when Vettel and Lewis retire.
The only way that would happen would be if Hamilton wins every season between now and his retirement (including his retirement year) or for Vettel to win the WDC again (in which case he's not going anywhere). Because if Verstappen, Leclerc or someone else wins a single WDC before then, they'll probably still be a WDC on the grid.
I can't see that happening. Every time Hamilton has won the WDC, he's seemed pretty hungry for another. Were he to match Schumacher's record in 2020 and beat it in 2021, he'd have to consider retiring having just won five WDCs in a row and looking like the firm favourite to win another. I don't think he'd do that - I can't see him retiring when he's at his peak.
During Hamilton's thirteen year career (sorry, I meant to say "during his 1.3 decades in F1"), he's seen two drivers retire when they're at their peak. He won't want to emulate Schumacher, who regretted his decision and had a disastrous comeback a few years later. And I think he'll be too proud to emulate Rosberg, especially after his comments accusing Rosberg for being a little bit cowardly for failing to defend his title!