-1, viruses are not sentient and are only given pressure to not kill their hosts if that reduces transmissability... which specifically doesn't apply for COVID, which is asymptomatically transmissable.
If you're dead you can't continue to transmit the virus, so lethality absolutely does decrease transmissibility. The fact that some people can be asymptomatic and contagious doesn't change that.
also if it was more deadly, presumably that would lead to people taking it more seriously.
E.g. HIV is asymptomatic for a long time, but is ultimately deadly, so people use condoms because of that (of course there are other STDs as well, but I feel like HIV plays the biggest role in condom use).
If its effects were less serious, it would probably be far more widespread (e.g. like HPV).
With covid more deadlines would probably lead to people would using masks more often, taking rapid treats more often, etc.