I am not a virologist but it sounds to me that this is an observation and not a law/axiom.

The virus does not decide a strategy, it is random mutation and selection and even if it is less likely it might be that we could have a variant that is super transmissible and super deadly. This effect could be obtain in the delay between transmission and visible effects on human body. Example: virus is transmissible with N days before we can see signs of infection.

Of course we can fight back by testing everyone weekly even if they show no signs.