> What's bad about them? There is a sensible default and users (in particular those with poor eyesight) will adjust them to their needs.

16pt is a sensible default on some screens and very much not on others.

> users (in particular those with poor eyesight) will adjust them to their needs

In the real world they won't though, much as we might both wish otherwise.

Guess what: When you setup your smartphone, it asks you what font size you want. So it's unlikely to be 16pt for everyone.