Tags vs Backlinks
I started thinking about this because Obsidian Publish doesn't render tags if they're in a note's frontmatter. So then I started thinking about the potential downsides of putting them in the body. But then I was reading the Foam documentation (I intend to eventually move my notes to my main site using Foam) and it mentioned that some people use backlinks instead of tags. That sounded strange, but when I thought about it, I realized that you actually could replace tags with backlinks if you wanted. The question is whether it's a good idea.
I don't think it is, because tags and backlinks served similar but distinct purposes. Tags are for broad categorization. Backlinks forge an explicit relationship between two notes.
Tags
Tags group notes under a broad category. They're essentially a better version of folders, at least from a GUI perspective. If you want to see all your cookie recipes, you don't have to hunt around for your cookies folder. You just search for #cookies.
The other reason tags are better than folders is that a note can only be in one folder at at time. But if we think of tags as folders, a note can be in as many folders as you want.
Nesting
One thing that tags do that backlinks can't is nesting. But that only matters if you think nesting is a good idea, and I'm not sure it is.
Backlinks
By contrast, backlinks draw a direct line from one note to another. They're more sophisticated than tags because they're more specific.
Tags come in handy when a note belongs to some obvious category (eg #recipes). But connecting notes via an obvious relationship is actually contrary to a notebook's primary purpose. We aren't trying to group things together by self-evident similarities. We're trying to tie ideas together, even if (especially if?) those ideas are seemingly disparate.
Summary
Tags are broad, backlinks are specific. They serve different purposes, so I don't think one can replace the other. You could replace tags with backlinks, but I don't want to risk diluting my backlinks with notes that are only broadly related. With backlinks, specificity is the point.