Earlier quoted context omitted.
So... Sounds like you want what I keep thinking about: Front end that works with feeds from multiple sources, like RSS, and permits comments, posts etc using SSO, with all interactions logged centrally in the SSO account (for later edits deletes etc). Sound about right? Because you me it seems like every provider in that list should be able to make a profit off the data and interactions... With users who want increas…
No SSO, because you're lurking without logging in
Essentially, Reddit Appollo and all the others are the front ends, with the subreddits being independent back ends (which would certainly also have their own front ends, even if restricted to their own sub).
Thus, the most popular front end would almost certainly just be a clone of old.reddit with the /all and /popular being made of the aggregate communities that front end has selected for default inclusion (with the most inclusive front end almost certainly "winning").
Tldr - SSO only matters for commenting/posting, lurkers would/could get literally the exact same experience as Reddit...