> A typical maintainer is going to say "wait, what; my accounting system is all about finance, none of this is relevant!"; but they miss out on what users really want in many cases: interoperability or data portability.

I think you are the one missing the point.

Users can want whatever they want to want. They're receiving, generally, et gratis et libre software which depends on someone else's time and effort. If they aren't getting what they want from the project, they are free to fork that project — and when they do, they're still getting more than they would otherwise be entitled to (i.e. the ability to start from scratch).

Further, a maintainer by definition is normally someone not charged with implementing new functionality (that's a "developer"), but simply with bugfixes.