I believe you would appreciate living somewhere else than in an American city, many other developed countries do not suffer from the same malaise as American cities, being dense doesn't mean "turning toxic", there are many ways to densify a city while keeping it livable.
> There is: tax dense office space. There are no magically unique "amenities" in dense cities that make life worth living there, for the vast majority of people. Theaters and museum visits are at most a monthly occurrence (if that). And specialty ethnic restaurants are usually not a deal-breaker.
> Everything else is now available online: specialty stores, interesting classes, art-house movie clubs, etc.
You live online, that's fine if you are happy, many other people actually prefer social interactions, meeting for shared activities, and cities provide these amenities and the population to find your groups around. There's no way around that, living like a hermit in the woods with your own workshops, watching films by yourself (or with your family), etc. is a lifestyle choice and you can do it either way if dense cities exist or not. People will still congregate around the same spots that provide access to the activities they want, they will live in cities that have cultural events (theatres, concerts, museums, art, clubs), that provide gathering places for like-minded individuals: sports, ceramic classes, painting workshops, musicians gathering for jams.
You are wishful thinking that technology will somehow replace a deep social need for many, self-driving is bollocks, distance to activities is what matters, trains already exist in many developed countries and they bring people from far away into cities and still there's a big demand for people to live closer to the cities. I'm not judging you for wanting to live like that but you are extrapolating a very narrow view of how people live into the broader picture which simply doesn't work.