Going to reply individually as each comment has different points and questions.
As for this one, I agree but I would also suggest that Reddit has some of the best UI tooling in the web's history for niche and large communities.
I don't know exactly where everyone goes when leaving reddit and I would imagine the answer is different for many groups and people.
In my experience, the largest reddit exodus was about 3 years ago when people began flocking to Lemmy in response to third-party API changes. I found nostr in that same window of time and thought building an alternative to Lemmy would be better. A year later I ended up using all of these options. Reddit, Lemmy, Nostr, and Pubky. Anecdotally my own experience defies your proposal. And all who chose Lemmy effectively are still using the same UI conceptually. I use Boost for Lemmy which is just Boost for Reddit as the developer also moved to Lemmy in response to the API changes.
Discord has a tight hold on many communities as it offers a combination of "resources and conversations." I still use Discord of course, as many of my own communities have lived there for over a decade now.
And so my point here is going to be- it doesn't matter where people go. The UI capabilities and the concept of sovereign communities are what bring users to a platform to create their own communities. What creates staying power is if the communities can evolve to become their own platforms.
None of the centralized services want that for us. Nostr and Lemmy offer it to some degree. Pubky forgot that platforms should cater to multiple communities but it works as it stands for isolated community platform software.
Which, all of this boils down to, "indexing on nostr is cooked and makes it really difficult to provide highly-available notes at competitive speeds". For now. And perhaps this is my own shortcomings, but I don't see a lot of forward momentum in this area.