Is extensibility really the Pandora's box the author makes it out to be? Just because something is extensible doesn't mean it has to be extended in monstrous ways.
It's interesting to think about this: a simple and ordered space is the most seductive platform to start messing with.
When something is complex and chaotic enough, we start losing the drive to make it even more complex, as it takes too much effort and it's too painful to do. But when a system is simple enough... you have ideas, you think them possible, you feel the opportunity, you want to experiment and explore and create; and it's hard to resist. Therefore, software systems (and many others, maybe we could spend a few thousand pages discussing this for societies as a whole) develop a tendency towards complexity. You could describe programmers as data manipulators or complexity managers. Don't make your first job harder, don't make your second one impossible.
Yeah I agree with you here. Gopher had Gopher+ that allowed for quite rich extensions - the original gopher authors had early prototypes of a 3D "VR" interface for example.
No one really used it for anything though, and then most people moved on as technology moved on.
I think perhaps the Gemini folks are acting slightly paranoid here - like the minute they allow extensions they will be flooded with ads and tracking. I am reminded of https://xkcd.com/538/ - I suspect that really no one cares enough to flood Gemini sites with ads and tracking, and likely never will.
