SSO only applies to those engaging, with those lurking being presented the default selection of subs for that front end site.

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...