Attritional interfaces
samgentle.com
samgentle.com
This means that, without constant activity, threads will eventually die. Third-party archives exist, but the chans do in fact delete old threads.
An individual person is naturally limited in the quantity (and quality) of their output, so the end result is a daily shortlist of original insights, essays, and articles that are manageable and worth reading. As a bonus, you no longer have to check up on individual sites, since everything will show up prominently in your feed.
This is why - despite having accounts in both - I follow Twitter and Tumblr feeds on my RSS reader. The sites are simply too limiting.