And this is exactly the reason why you need vendor support to apply fixes. Half of those fixes are in binary blobs, so if you don't update those (e.g you rely only on AOSP/Lineage updates), you're screwed.
> Monthly security updates to be supported for at least 3 years after initial phone release.
(scroll down to the bottom of the page and look for the double asterisks footnote. I don't know why they want to hide what is IMO the best reason to get an Android One device)
It's still a short window IMHO, and it's bad for the planet to throw working devices after 3 years.