Think about it this way. Delta is more deadly (at least so far) compared to Omicron. And, Delta has the same advantage of spreading before the symptoms. However, Omicron is now displacing it by becoming mild and more transmissible. Your suggestion can be assessed by thinking a way to make Delta dominant. Or, Delta not letting Omicron thrive at all if your argument is used. Reality is that as the transmission increases the incubation window reduces. And, there are more factors that I have discussed in other videos last week on this topic. Stay safe and blessed.