Old Buffalo said:
Weird. It's almost like we know viruses don't mutate into more deadly strains. Like through science or something.
So that's not exactly a rule, and like everything else for the last year taking some generally true case and building a polemic wall out of it is a bad idea.
Viruses are collections of genes. The collections that are better at replicating and spreading between hosts will win out in the evolutionary race. So generally, really deadly viruses are at a disadvantage because they kill hosts before they can spread. COVID is already pretty non-lethal, and critically takes a long time to make people sick (and for them to die). That's the main reason it spreads so well. There is plenty of evolutionary space for it to become more lethal without reducing its infectivity.
Do I think super-Ebola-Covid is just around the corner? No. I don't see any evolutionary pressure that would favor it becoming more lethal, so it would have to happen by chance. Also it may not even be possible, as the evolutionary path it's taken might dead end without possible virulence enhancing mutants being available.
But, unlike say Ebola, it's not impossible for a new Covid strain to be more deadly.