Yes, I believe the high transmissibility before symptoms appear is the most beneficial trait to Omicron. However, if a variant appeared with exactly the same traits except that it was 10x more deadly, then a lot more people would be taking it more seriously, self-isolating and driving down infections. From an evolutionary perspective, there is an incentive for the virus to become milder rather than more deadly because of how the news of a deadly virus changes human behaviour.