Jack of all trades master of none fullstack devs are rarer the higher up seniority you go. They are much more likely to become people managers given the way influence is wielded at a big company. I can see them at smaller companies, but it sounds like a waste of potential.

In my experience it's the exact opposite. Junior devs can only handle small parts of the stack at once, but more senior people can handle everything. Senior people specialize by gaining domain knowledge.

The senior people can still handle as much as the stack as they could as juniors, but they have deep experience in a few areas where they are considered experts.