bee
This is the answer, and for anyone who hasn't played Outer Wilds it's still their answer and they just don't know it yet.
Honestly, at this point, it doesn't matter to me. After ~12 years of Reddit, I was looking for an excuse to leave, and they finally gave me one. Even if the Fediverse is never as popular as Reddit, it's already enough that I don't feel the need to go back.
They told me I could become anything so I became a bee
If reddit simply wanted to sustain its userbase, it'd be in a much better place.
That's fundamentally the problem with the internet (and corporate culture as a whole, really). When a site takes off, the expectation isn't to sustain the status quo, it's to continue that growth. It inevitably reaches a point where maintaining exponential growth becomes more and more difficult, but even more profit still needs to be made. They find ways to squeeze every last goddamn drop out of it, almost always to the detriment of the users, the user's experience, and everyone's privacy.
The never-ending growth model is fundamentally the problem here, and that's why I completely agree with you. Like you said, "This requirement is incompatible with profit motive."
One of the things that bugged me the most about Reddit was how it felt like everyone was constantly engaged in this game of one-upsmanship to try and seem like they're the smartest but also the most cynical. I don't get that vibe here at all and it's really exciting.
You can see it here. Absolutely gorgeous guitar.