While true that younger people are less at risk than older people, that risk is non-zero and significant.
You also ignore that students don't isolate themselves within the walls of the college. They are a part of that community, using dining and retail facilities, traveling home, visiting nearby cities, etc. That puts their parents and families at risk, locals at risk, etc.
Not to mention the majority of professors would be considered at least moderate risk. Maybe I just had a particularly good experience but I got to interact with a number of profs quite a bit during undergrad. Paying for in-person with only a bunch of TAs just wouldn't be worth it IMO, plus things like campus events would also have to be limited. It would take most of the benefits of the college experience out of the picture. I think gap year is the way to go for anyone that can make it work, regardless of whether the schools try to do online only or make accomodations for in-person.
You also ignore that students don't isolate themselves within the walls of the college. They are a part of that community, using dining and retail facilities, traveling home, visiting nearby cities, etc. That puts their parents and families at risk, locals at risk, etc.