A good point. The less aggressive Covid-19 type S virus may be the dominant form for the present. But it is probably not the last sub-type we'll see. More mutations should be expected, as this newly emerged virus adapts to human hosts world wide. But, as always, natural selection will be at work. Any advantages offered by new mutations will be favored, but only in combination with the selective pressures created by medical & public health efforts from the human hosts of the infectious virus.
By
selective pressures or
evolutionary pressure I mean something specific.
In the lab, if someone is studying the function of a particular gene in bacteria, they alter that gene to see what happens to the bacteria when it can't work. What if that mutated gene makes the bacterial grow slowly or not at all? Would you be able to find it growing among many other bacteria that didn't have the mutation? You physically attach that mutated gene to another gene that creates resistance to an antibiotic such as penicillin. When you grow the bacteria on nutrient plates containing penicillin, only the ones with the penicillin resistance gene can grow. Among them will be the interesting mutations of the gene you're interested in studying. That's only one commonly used selection method. There are many others.
We can't do that with humans, but we can still talk about selective pressures. The efforts at containing contagious viral diseases by public health measures such as quarantine, or by medical interventions, can affect whether new mutations thrive or fail. That also qualifies as selective pressure.