My 2-cents :) I have created several state graphs for languages under Joe, and while it took some getting used to, the system itself doesn't have to change with the changing landscape, just add another syntax.
Note that Scintilla (the syntax-highlighting and editing component that Geany uses) is so much integrated into the editor (and plugins) that it would be really hard to replace it with something else and I don't think there are any plans to do that. So Geany will always depend on the availability of Scintilla lexers for language support.
That said, I don't think there's any urgent problem on Geany's side even with the current hard-coded language support. The fact that I added support for several languages recently was driven by my feeling that some language support requests have been neglected for a long time and I tried to fix that - and the currently open PRs are all I plan, there's nothing else in my sleeve. And I think it will be always the case that some "esoteric" languages (and of course what's esoteric depends on who you ask) will not be supported.