If anything, most viruses (that we know about) mutate to make themselves more "potent". That's just the nature of their relatively simple programming, as far as life forms go, and viruses don't even follow some of the rules which define "life forms". So who knows?
But I hadn't considered that it's the patient base, not the virus itself, that has changed. The explanation that most of those people more succeptible have already been hit seems to fit somewhat with earlier reports (from like several weeks ago) saying it could be that more people are asymptomatic or only mildly affected than we realized. The first wave takes out the most succeptible. Following waves come, but we don't see as high a level of critical cases because the "weakest" have already been culled. And there are many that will never show any symptoms at all, due to their personal physiology and DNA. Herd immunity achieved the hard way, I suppose.