> the compiler will guide you towards the completeness of all the functions in your codebase that match over your type. So this burden is not on the programmer.

That only works if all the functions you care about are in one codebase. If not, then the changes the article talks about become backwards incompatible changes in your library.

> So I guess these are not real problems.

If you're designing a library, the approach you take determines what types of additions are breaking changes and which are not.