It's important to note that these "8 years" aren't actually that in practice due to the delays. The latest generations of Pixels (starting with the 8th) have 7 years of actual security updates, which is one year less, but is proper support. Hopefully the industry trends towards that as a whole - buying devices that only get 2-3 years of updates should be a thing of the past.

Isn't that related to how expensive it is to get licenses from MTK and Qualcomm for updates? Given that Pixels run on their "own" chip, driver support is probably much easier.