So if I'm looking for a solution rather than a library or a project to take part in, I need to search a dozen sources to see if anything meets my requirements? What if the project was just abandoned without being published any of those source registries? What if the project was a PoC of some algorithm that didn't need to be published anywhere else?

That sounds hardly like an alternative to what's possible with Github now. The only alternative that came anywhere close to that ideal was freshmeat - and even that didn't achieve the full potential. Check this discussion alone to see how many talk about 'network effects' or 'discoverability'.

The subject is decentralization. There’s a huge value in curation and specific communities. You don’t go to r/all to read about emacs or bash. Instead you go to r/emacs and r/bash. Even those “awesome $thing” list are better than going through pages of GitHub search results.