Or /r/all without the NSFW subreddits, which ends up being indistinguishable from the default.
That's not true. There are only around 20 default subreddits. r/all is everything.
Yeah, and nearly everything on the front page of /r/all comes from those 20 subreddits plus a few NSFW subreddits. The smaller subreddits probably don't even want front page exposure.