"I started by removing the Page List block from the Navigation menu in the header. I have 90+ pages on my install, and it is always irritating when themes list them all by default."

When creating a theme it is difficult to know which way the wind is blowing with the Navigation. There was briefly the navigation areas (primary, secondary, and tertiary) that solved a lot of issues and worked quite well, but no sooner had I started working with it, it was decided to deprecate it.

I don't like leaving the default menu empty, nor do I like using hardcoded "demo" links. The Page List works well on a fresh install, but like you say with a lot of pages it really doesn't!

I seem to remember some classic themes submitted to the directory would not have a fallback menu so the initial state was empty, and the theme reviewer(s) would often hold back on approving the theme until the author had addressed this. Usually they were encouraged to output all pages as the fallback.

At the moment the Page List, although not ideal, seems the best compromise. A small snippet of CSS could limit the displayed page list menu items, but when we want to get away from themes adding stuff like this and let core/gutenberg handle it, a maximum number option would be a great thing to have.