[sterlind]

How do you arrive at several decades and billions of organisms? You can't extrapolate the current mutation rate to the spillover event; SARS-CoV-2 was already quite fit at infecting humans and didn't need to change much. In contrast, during spillover events there's heavy selection pressure to adapt to the new host.

There's also the possibility of recombination events, which can instantly cause significant drift. Now consider the case of the Mojiang miners, who contracted and slowly succumbed to viral pneumonia in 2012 after inhaling bat guano. WIV found, like, a dozen beta-coronaviruses in that cave alone (including RaTG13.) The miners could have been exposed to several at once, which recombined and adapted quickly to their new home in human lungs. Again, unlikely to have been SARS-CoV-2, since the pandemic almost certainly would have happened already, but whatever they had was virulent enough to kill half of them.

[lamontcg]

The genetic clock was estimated from bat coronavirus mutation rates in the wild, not from SARS-CoV-2 mutation in humans.

And virulence != transmissible. They got sick by inhaling everything that was floating around in that mine. To get more people sick you'd need to take them all to that mine.

Recombination to produce a viable and more successful virus is also very rare, that is unlikely to have happened to any of those miners, much less several of them (over populations it becomes likely but there we're talking many millions of infections as well).