The Wikipedia apps for Android and iOS offer features that are (needless to say) specific to Wikipedia sites. However, the actions themselves are pretty generic for a MediaWiki site (read content, save pages, etc). In the backend, the APIs used are basically equivalent.
Imagine that those apps would have a generic codebase, and that the Wikipedia specific elements would be added in a configuration file. Imagine that owners of other wikis could just fork that code, edit the configuration file to point to their wikis, and package their own Android and iOS apps.
No maintenance promises, no support promises, just a new release for each MediaWiki release every six months.
The benefit would be more users of those open source apps, leading to more potential feedback and contributions.