I think under the contraints of the reddit system, you're absolutely right. Moderation is required to maintain quality and prevent dilution from spamming, trolls, etc.

But I'd suggest that's again because of reddit's dynamics.

Consider a different social network that is instead focused on enabling users to group with other users they deem productive (automatically, simply through interactions, or manually they can give a score from 0-100 for each person [otherwise this score is auto-generated based on signs that indicate a productive relationship). So in this social network, each person's content is effectively cooperatively curated by people they deem productive members of the network.

I believe under a social network system like this, that moderators would not be necessary. Yes the social network, as a whole, will get spammed, trolled and diluted with many many posts. But there's other systems for helping navigate this. Ultimately though, I believe the concept of productive networks supersedes the need for most moderating functions. Will there still be a need to prevent stuff like death threats or etc. etc.? Yes absolutely, but I believe that decision is ultimately an admin-level decision, not a moderator-level decision.