Wrong.

It doesn't matter if the host dies or not, all that matters is that it can spread before the host dies. COVID was perfectly capable of doing this via asymptomatic spread. (see: Delta).

Omicron outcompeted Delta because it had mutated so wildly that it could bypass all the antibodies the vaccines had generated up to that point, plus it drastically reduced the incubation time, meaning more spread potential. That's it. It could've been as lethal as Delta and would've still been successful.

Omicron is likely as severe as the original strain, the difference is now everyone has some kind of immunity to it, so it wasn't killing people nearly as much on a per-person basis.