Even on public instances, I don’t understand why people think defederating hurts the fediverse. Just join a different instance, how hard can it be?

I’m on 3 different instances right now

Social media needs to be as easy as possible if you’re going to reach the masses. Most people do not give a shit enough to create 3 accounts; they’ll just leave.

That’s just fine with me. Quality > quantity

Some of the best forums I use have just a handful of active users

But quantity and quality are linked. If only, say, 0.1% of people will post high quality content, that means you need to attract a thousand people to get a high quality poster. You can’t just put up a sign that says “high quality posters only”. Plenty of quality posters also want an audience, so they’ll go where the people are and leave if that audience isn’t there.
This. I love reddit r/askhistorians and r/askphilosophy. But the vast fkn majority of people are not qualified to answer historical or philosophical questions. In these cases you need a lot of people on the site.