Some comments are saying that the 6-year LTS is needed to support older Android devices. Also, in practice most vendors don't bother releasing updates to phones after the first 2 years, other than security updates.
One possible nice side-effect of not maintaining kernels for so long and allowing people to stay on out-of-date systems would be to encourage vendors to allow users to upgrade to the newer versions of Android for more than the current 2 year life span. They are then more likely to put pressure on their component vendors to get kernel support for their chipsets into mainline so they don't have the excuse that they can't provide updates because the hardware isn't support by modern firmware.
One possible nice side-effect of not maintaining kernels for so long and allowing people to stay on out-of-date systems would be to encourage vendors to allow users to upgrade to the newer versions of Android for more than the current 2 year life span. They are then more likely to put pressure on their component vendors to get kernel support for their chipsets into mainline so they don't have the excuse that they can't provide updates because the hardware isn't support by modern firmware.