What criteria do you use for deciding what to archive and what to delete?
In our case, it's what I said earlier: the decision is based on (1) traffic, (2) time required for maintenance, and (3) whether the pages are relevant to our main topics or are random pages that once served a function but now seem unnecessary.
The importance of (3) shouldn't be underestimated. Sites evolve over time, and so do content or publication strategies.
When our current site was first launched as two previous sites in 1997, it had similarities to a magazine, with new articles each week on a wide variety of subtopics under our broad main topic. (Many blogs today have that same publishing approach.)
Over the years, our site has evolved to the point where it's largely coalesced around a half-dozen or so major subtopics, so a lot of those old evergreen articles on this or that could be termed "editorial outliers" on the site of today
Just as important, other niche sites have come along to cover those "editorial outlier" subtopics much better than we do, so there's no longer a compelling reason for us to maintain articles on minor subtopics that don't especially interest us or most of our readers.
Since we aren't supposed to use real-life examples here, let me use a made-up example that has nothing to do with us: baked goods.
Imagine that, in 1997, we had a lot of content about doughnuts and muffins. We were still in a magazine mindset, so we wanted to do something new every week. Because there weren't a whole lot of Web sites about breakfast sausage and bacon, we published a number of small articles about those and other breakfast topics while continuing to build our doughnuts and muffins sections into major, authoritative sites. Nowadays, most of our traffic is on our doughnuts and muffins sections, and other people's dedicated sites about breakfast sausage, bacon, coffee, etc. have far more information and insights about those non-doughnut/non-muffin topics than we could ever hope to publish (or would want to publish). So it makes sense for us to get rid of the articles about breakfast sausage, bacon, coffee, etc. and focus on the topics that we know and own: doughnuts and muffins.