I agree with everything you've said except with comparing viruses doing us in with mammals. Mammals did not do the dinos in. Something else did. Some say it was a big rock from the sky. I wasn't there so cannot confirm. But I am almost 100% sure it was not the mammals. They seized the opportunity, that's for sure. A virulent disease cannot do that. It is good as a scary bogeyman and definitely a bad news for us, but a disease that wipes out all its hosts will not get too far. At best it may act as the modern equivalent of the rock from the sky, to pave the way for whoever comes after us. It won't be the virus.