Why did the old web die?
I'm a HN Heretic. HN says dark patterns, money, power, corporate interests.
I think it was very simple: Proprietary platforms solve real-world problems the more "open" web doesn't and did not effectively solve: discoverability, spam filtering, content filtering, community. Regular people don't want the open web, and never have. They only tolerated it when it was given to them without alternatives.
Proprietary platform solve all the problems you cite for exactly as long as it remains profitable to do so, and not a nanosecond longer. Once you’ve been captured, say goodbye to every one of those things.
That's not new; it's been happening all the way back since AOL. AOL was basically an abstraction layer over the whole internet that we tolerated (remember when every show had both a URL and an AOL keyword?), but it broke like anything can.
For that matter, your maxim also applies to the open internet, and watch what's happening. It's not profitable, so sites are packing up.
In a nutshell, content costs money. People make content anticipating money. Doesn't matter if it's on Discord, on YouTube, or a private blog. No money, no investment.
Also it is easier to publish to a walled platform than the open web.