I've definitely drifted away from a number of alternative tools/platforms/spaces over the years because I was never really able to connect with them sustainably for one reason or another as well. And you're absolutely right that remaining engaged with a platform like SSB requires unlearning deep-set habits and expectations for a lot of people - an effort which many will ultimately conclude is not worth the effort.
However, I would emphasize that one crucial factor in that equation is the extent to which newcomers are able to meaningfully connect with other people in the community which they're seeking to join. If you develop strong enough bonds with even just one person in a particular space, you're much more likely to become a long-term community member than if you don't really feel like you connect with anyone or are valued (similarly to moving to a new town or starting a new job, for instance).
One of the reasons that I've personally become so enamored with SSB in particular is because there is a powerful, tangible sense of accessible community, history, and culture throughout the platform which I find so often lacking on Social Media. The Scuttleverse is not sprawling and addictive in the way that we've been trained to expect 'successful' social platforms to be, but it has a warm atmosphere and its roots are deep, strong, & ever-growing. Getting back to the analogy of the 'web city' (and setting aside the fact that SSB technically need not be attached to the web), it feels a bit like a beloved neighbourhood café which may never be able to compete with 'Starbucks' - but maybe that's not really the point. SSB's structure and values framing is fundamentally different from that of towering platforms like Facebook & Twitter, which is precisely why a particular core community of people have continued to nurture it over the past few years.
Prior to joining the SSB community, I've explored many other alternative social platforms (such as Diaspora, Mastodon, Matrix, etc.) - but never felt particularly welcome or valued as a newcomer. And that's the main reason I drifted away. Trying to become 'significant' enough to forge connections felt all-too-similar to the struggles I've experienced on major Social Media platforms. In stark contrast, I was very quickly and warmly welcomed into the Scuttleverse when I arrived there by a bunch of friendly, interesting people (often engaging at length with one another respectfully in longform!), and I now do my best to carry on the broader tradition of what butts refer to as 'community gardening' (loosely, fostering healthy social engagement & inclusion) there myself.
Similarly, you and I are currently engaging in a type of conversation which strikes me as an emblematic of this same emphasis on qualitative, open connection with unknown humans. Despite having no previous association with one another, you reached out to me through Write.as, and invited me to have a conversation through Are.na in a way which explores a new area of potential beyond the norms of online dialogue. In other words, you took the time to reach out to a newcomer to your community and created space for me to speak with you openly & at length in a completely different ecosystem (beyond the walls of social metricization & commodification). When does this happen on platforms like Facebook or Twitter?
[I'm now put in mind of the fact that, in a way, we're also engaging in an alternative, more DIY version of Letter--which arose to meet a deep desire which many Twitter users felt for more qualitative open human exchanges than the platform allows for, and has grown strong because it offers a novel framework which meets those values (though, sadly, they've recently introduced the number of "Reads" as a prominent default metric).]
Ultimately, I would say that whether it be Write.as or Are.na or Scuttlebutt, the long-term viability of alternative / open "y" platforms depends upon the extent to which they resonate with / respond to the values, needs, & growth of a strong core community of people who are weary of the monopolizing / restrictive "X" Platforms--enabling the formation of new habits & norms of online engagement/community-building in the process. This is, I think, less about direct competition as about finding & making room for greater online diversity and autonomy.