I think this is spot on. The viruses don’t become miraculously less harmful. At times it happens, but there are reasons for that (for example if a virus has one strain that kills you fast, and another that just sickens you a bit, the second one will eventually dominate and so the virus has become less harmful). Covid seems to be extremely efficient at infecting people, to the point that many don’t even know that they are infected and some others know it after a week. For it to spread fast, it does not need to become less harmful.
Many viruses never became less harmful. Typically they have a very high R0. Measles and smallpox for example are two of the viruses that spread most efficiently, and they never became less harmful. Some might even become more harmful (H1N1 in 2018) but there has been a good reason for that (considering that the first wave was mostly on soldiers, those that got mild symptoms just stayed on their room, while those that were real sick had to be transported to hospitals, which in the war involved a contact with many people and so the more harmful strain came to dominate and result with the second wave).
So while on average, the virus might become less harmful over the time, each virus should be considered in isolation, and if it becomes less/more harmful or stays as it is, it probably depends on R0 among other factors. Probably covid does not need to become less harmful considering the efficiency it spreads. The current strain(s) can continue spreading without needing to heavily mutate.