There are definitely pictures of packed protests. All I can say is that the ones I went to were socially distant, except one march in San Fransisco, where that cohesion was lost early on.

And while there were no doubt some people with masks at Trump rallies, many were not. Which is why you had more spread as a result of some of those rallies compared to the protests.

It's also possible the changing weather played a role, of course.

As for the life of an 80 year old vs the livelihood of a 40 year old... That's a false choice, in the United States, at least.