Doom and gloom prediction could pan out IMO. The vaccine won't work against the new mutant strain of corona which recently emerged in the UK and new york.
The prediction certainly could turn out to be true, but I don't think coronavirus is going to be the ultimate reason. This bull market in stocks has been going on since the banking crisis of 2008/09, and the laws of financial gravity don't allow for any asset class to just keep going up and up indefinitely.
On the other hand, popping bubbles are invariably precipitated by some sort of catalyst, but my guess is that there's going to be some kind of increase in inflation and/or interest rates that kicks it all off (and the effects of COVID-19 won't help matters, either).
I wouldn't worry about new mutant strains and vaccine effectiveness, however. Science will keep fighting the virus regardless of what it does, and unless it suddenly turns out to be far deadlier than it is, I'm not going to fret about it.