grandMattie I don't understand your logic. It isn't inevitable that viruses become milder. Evolution just doesn't work like that. Viruses are dead. They don't have any "aim". Mutations are entirely random. The most transmissible ones will survive because they'll latch on to a host more readily. Some of them will be more deadly (as the delta variant is) and some not. Viruses just don't think strategically - if they kill all their hosts, so be it, but I hope we never get to that stage.
Viruses don't mutate as a result of running out of hosts. Every time a virus reproduces itself, there's a chance there will be a mutation. Some of the mutations make the virus non-viable and they disappear, but some are more serious and the more transmission there is, the more likely there will be variants which overtake vaccines.
One of the reasons that Covid-19 cannot be treated like the common cold is that colds don't usually leave people with after-effects. People might be quite ill for a while, but usually they recover fully. Covid-19 just isn't like that. Long Covid is, thank goodness, now taken seriously and the NHS has opened treatment hubs, but to date nobody knows how long the after-effects will last. A handful of people, like Derek Draper and Michael Rosen and thousands of less well-known people, have life-changing disabilities as a result of Covid.