I think we are talking past each other. I think that the virus has changed, yes. What I am trying to understand is 'What does it mean when a virus variant where the secondary rate of infection is indistinguishable from a prior rate of infection is infecting a whole lot more people than the first variant'?

One possibility is that it is a whole lot milder, perhaps especially among the vaccinated. People get sick, and instead of feeling awful, and lying in bed at home and heading to the hospital, they are out and about their daily lives as if nothing has happened, because for them nothing very much has. You aren't more likely to catch alpha than delta, for the same amount of time of exposure to a sick person, you just are much more likely to meet a person with delta than with alpha. The alpha sufferers are all at home in bed.

But you could imagine a world where the delta variant was the first variant of the disease that started infecting people, and it went around the world until only those resistant to it were left uninfected. And then, randomly, the alpha variant shows up. Then what happens?

one possibility is -- nothing. The alpha variant doesn't spread at all. It is less successful than the delta variant, therefore any population in which delta isn't spreading (because the disease cannot find enough new people to infect) isn't going to spread the alpha variant either. This is the pattern for a disease that is about to die out, globally. No matter how the virus mutates, it still cannot get established again. The human immune system beats the virus here.

A different possibility is that the alpha takes off and starts infecting tons of people. This means that 'delta resistence' is different than 'alpha resistence'. If the people who are catching it also have milder symptoms, either because they have just taken a vaccine that is designed to prevent serious illness and death, or because the people who were for some reason more likely to get serious illness and death have already become sick, they will travel all around, infecting others and not staying in their bedrooms. Because, after all, they aren't feeling sick.

If the second is true, then whatever genetic changes the new variant has are less important in making people less sick than 'a close viral relative to me has been raging through here last year'. And we have no reason to be particularly hopeful that the whole disease will burn itself out. It could be like flu -- compared to the early days, almost nobody gets seriously ill and dies from it, but every year there is a new variant and some people will catch and spread it, and this continues forever. (Unless the people trying to develop a vaccine that will work on all flu variants are successful.)

I figure than both factors are