Think about it this way: what is the evolutionary pressure for it to become more lethal? Particularly if we accept your premise that such lethality occurs after

Think about it this way: what is the evolutionary pressure for it to become more lethal? Particularly if we accept your premise that such lethality occurs after it spreads. Now a given strain may mutate to become more deadly but why would such a strain become dominant? Even if all this occurs after the virus can spread, consider the human reaction to it. You have two diseases: one is essentially the common cold, the other is basically Ebola. Even if those events occur after transmission, the response to each disease is going to be wildly different. The common cold will just be tolerated and managed. There will be no lockdowns. The second will have the military enforcing lockdowns and curfews combined with hard travel borders and checkpoints. There would be forced vaccination in some places and severe restrictions on the voluntarily unvaccinated. Just the human reaction alone will tend to make the common cold version more dominant over a sufficiently long time frame.