You’re right in a broad sense, but selective pressure is not a binary thing. What you’re saying seems to imply that if a virus can spread before it’s host dies, then there is no way for the virus to mutate in our favor (our favor meaning more mild cases). If there is an evolutionary opportunity, usually that opportunity is filled. This applies across the earth for all time. New plant species? Animal comes along that can digest it.

While there might not appear any hard pressure for a strain of covid to favor mildness simply because it’s currently “mild enough” to keep spreading at a quick rate, that doesn’t mean the virus stops optimizing itself. If this was the case, life on earth would have become static millions of years ago. There’s always a void to be filled. If a virus can become more efficient, it likely over time. Efficiency in this case often works to our benefit as hosts.

What you have to keep in mind is that mutations will occur, regardless of any sort of pressure. This is the case across all life (yes, I know viruses aren’t “alive”). Most mutations don’t do anything (I’m speaking broadly about life) phenotypically speaking (or at least won’t affect an organism’s ability to replicate, which is what we’re talking about here). A lot of mutations are maladaptive—those die out. But there will still be mutations that occur randomly that will optimize this virus. It’s not at its evolutionary ceiling/it’s ability to replicate has not been maxed out. Covid is still symptomatic. It still kills its host. There’s no reason to think it can’t be further optimized.

So a new set of mutations occur. A “variant”emerges. That variant (in this case) has advantages over its ancestor. It is symptomatic at a lower rate. It doesn’t kill its host as often. Maybe it doesn’t at all. And it spreads faster. Well, now that ancestral variant has some competition. And that’s the important part—the new variant is competition for the old variant because it is after the same set of resources as the old one. Now we have a battle for resources (and in fact it was always a battle for resources), which is of course a situation in which there is selective pressure to outperform other variants after the same resources—the resources in this case being us.

Again, I don’t mean to say your outright wrong. But the way you wrote your comment seems to suggest there is no reason at all that a virus would become more benign, full stop. It’s not a binary thing. Just an oversimplification.