That's not how mutations work.
If the virus mutated in a single person and that mutation was "weaker" that strain of the virus would die out pretty quickly.
It's not like the virus would suddenly mutate in all hosts at the same time and we'd be left with a new weaker strain that was tolerable.
Survival of the fittest... even among viruses... mutations that aren't advantageous tend to die out pretty quickly (unless something other than natural selection is at play).
I don't know viruses, but I know evolution; and it isn't actually "survival of the fittest", it's "survival of those that are best at passing on genes".
A virus that doesn't immobilize or kill its host will become much more prevalent in a population. Covid-19 is basically going to be extinct after a year, but for the time being, the strains that allow people to spread it quicker will survive. It should tend towards a similar effect as influenza, but it won't have the opportunity to get to that point, I would imagine.