It's the popularity of a thread, not its length, which keeps it at the top of the first index page so making a thread a sticky will only guarantee it remains at the top of the page and if too many are stickied, any new threads will be forced to appear at the bottom of the page - or maybe even on the second page.
By remaining unstickied, threads will eventually get pushed down the index page by newer, more popular threads.
Even some of the long-lived games threads have had their quiet periods and floated off onto the second or third index page.
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Giraffe: a ruminant with a view.