This isn't really my area so this is conjecture and guesswork.

If various immunocompromised people get infected then by a fluke their immune system could be just powerful enough to contain the infection but not eradicate it, this will remain the case until the virus becomes too powerful and the host dies.

It's not the absence of selective pressure that is driving the mutation here, it's the mere fact that the virus might survive the selective pressure that does it.

Also from some of the explanations I've read of Covid-19 the mere duplication of the virus isn't all that damaging, the harsh way the immune system eradicates infected cells is more damaging. So someone with a limited immune response might survive for a surprisingly long time.