this behavior has been seen a lot of times in previous pandemics where once very severe pathogens end up becoming much less severe but also more contagious in the process
I’m missing the OP’s first message as context, but just a note that diseases don’t really have any particular trend toward less severe forms. That they would naturally do that was a debunked theory from the 1800s. They can’t simply kill or disable their hosts before they get a chance to spread, but most illnesses have plenty of room to transmit before symptoms get to that level and viruses frequent evolve into more deadly forms.