This is why I so strongly believe we need moderation/curation.

But let's consider the problem from a different perspective: what if tags did not exist?

If tags did not exist, we would only be able to find new content by following people. This is how real life has worked for millions of years, so our ingrained social rules will probably.

Now, it's obviously implausible to directly follow everyone who might post content on a subject you're interested in, or even follow everyone who is close enough to share it. Which is where a degree of implicit trust comes in. Some magic formula (left as an exercise for the reader) takes into account indirect follow chains and (to a lesser degree) simple share/upvote chains.

New users can still enter this ecosystem. As soon as someone upvotes one of their comments, they gain a little implicit trust so other people can see their post.

Now the only question is: where do tags fit in? I'm sure everyone claims that they are useful, but how?

Unlike subreddits, tags are not the primary way I discover content (for me at least). And most of the tags I do see in my feed I added because I saw people I follow posting in them.

I think that tags are only useful as a way to filter posts that I'm already seeing for the sake of seeing a longer history.

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Yes, I am aware that this will lead to a bit of a walled garden effect. But, remember that it only removes the visibility of posts, not comments (unless you block them of course).