tl;dr: There is a binary choice between a protocol being inextensible, or that protocol ending up controlled by a small number of Big Tech organisations.
In the early days of Gemini, much was made of the notion that an implementation could be completed in a weekend. A statement to this effect still stands in the FAQ on the geminiprotocol.net capsule. There was also at the time a lot of controversy, some of it confected, about the extensibility of Gemini.
What I hadn't spotted at the time was how the two issues were linked. I was more concerned to work out a good counter-argument to the reasonable point "why not just use a subset of the WWW?". But it is only recently that I spotted the importance of the relationship of extensibility with keeping Gemini within the scope of what could be done in a weekend.
Essentially it is this:
Of course the tacit bit isn't always true: it won't always be the case that if a protocol allows extension it will necessarily experience the hypertrophy characteristic of the web or email. But there could well be strong forces propelling a putative future protocol in that direction, and it is no comfort that it has taken 35 years for the web to deteriorate to its current condition of unseemliness.
There's nothing special about the "person-weekend" benchmark as far as this argument is concerned: over time, any extensible protocol will expand to the point where only a handful of well-resourced tech organisations can create an implementation. This is what has happened to the web, indeed, it's not even clear that any organisation can create a new web browser de novo any more.
There is a binary choice: either a protocol is not extensible, or it will end up being controlled by a small number of Big Tech organisations.
Anyone pushing to jemmy a bit of extensibility into Gemini or any other smolnet project is basically saying that the little people shouldn't be writing their own protocols and should leave it to the oligarchy.
(I use protocol above as shorthand for a thing like Gemini, WWW, Email, IRC, SSH and so on, some of which are more than merely protocols).