This is a great point. It feels to me like a chapter of the internet is closing before our eyes. Sites like Twitter and Reddit were built on the community spirit of the early internet but had consolidated a the traffic and content into a centralized platform. It seems to totally elude the tech bros who are currently running these companies that the community spirit is the primary thing that makes the site worth going to.
Community stuff is rewarding but also a giant pain in the ass - if you've ever been to a government hearing, a PTA meeting, or even a soccer team listserv you know that dealing with community is full of difficult interactions with difficult people. So it's not like the mods or the 10% of users that make most of the content are ever going to be easy to deal with, nor is figuring out how to monetize that activity all that easy.
I'd bet that Reddit is facing intense pressure from its investors to give them an exit. Those investors look at Twitter and see that nobody had a financial model that would sustain the site without destroying the community and understand that the same problem applies to Reddit. MetaFace quietly pivoted from being a community content site to being a user tracking and profiling business and built that up in time to become a duopoly with Google. Reddit is too late to claw out a piece of the user profiling business. I'd bet the investors want to create a metrics-based story about growth that they can sell to Wall Street Analysts to hype the IPO and cash out on the backs of less tech-focused investors.
What you are talking about entails running the business for sustainability instead of growth. VCs don't do that, and people like /spez in that circle are living in that VC mindset or may still be drinking the late 90s/early aughts Kool-Aid that they can marry a community-oriented site with hypergrowth.
I'd guess the site will look like Tumblr within a couple years. I don't really see a centralized alternative emerging in the near term. It may bring traffic back to Slashdot though.