No virologist or epidemiologist will say it is 'usual' that viruses tend to mutate to become less virulent. Perhaps, and even that is contentious, an expert could make a claim for a specific type of virus.

This type of reasoning is based on an assumption that a pathogen limits its reproductive number by increasing its virulence. It is reasonable to test this hypothesis, but unfortunately we do not have enough data points to make any conclusive statements.