Prior coronavirus species tend to evolve toward non-lethality because their peak transmission coincides with peak symptoms. Covid-19 is not like that and so has no evolutionary push towards non-lethality.

A variant of COVID as lethal as MERS would spread just as well since carriers are asymptomatic most of the time they are infecting others.

I agree that asymptomatic transmission reduces this pressure, but symptomatic SARS-CoV-2 carriers are often still contagious, and there is more public will to enact extreme precautions for more lethal illnesses.

So there is still some evolutionary benefit for lower lethality.

I'm trying not to read too much into the new variant yet, there isnt enough data to form a meaningful conclusion. But hope springs eternal. ;-)