That is kind of how .AI started out, some talk here and there, small sales, a few years go by and then boom. No longer a niche thing.
.SI taking off as an extension for Super Intelligence would not even come close to the craziest things I've seen in 25 years of domaining. Actually after seeing the rise of .AI it would kind of make sense in hindsight if it did. AI is still in a very early stage of its growth.
.SI may be a nothing burger, or if it does half or a quarter as good as .AI then that would still be big. A lot of AI and SI products and services will be coming in the next years and a lot of them will need a domain. The tech industry hasn't been shy of using all kinds of extensions like .ai, .io, .co, .xyz, and so on.
It's an open extension and registrations are very cheap compared to .AI, LLL and one words are still available, so it's not a huge investment to register a handful of domains as a low risk, high reward strategy. Spending a couple hours to hand reg the best 10 or 20 names still available is not going to break anyone, and could result in a x,xxx or xx,xxx sale down the road.