I think the discussion about which devices live longer is simple to answer: Apple (iPhone) and Google (Nexus/Pixel) do probably the best job of supporting their devices for a while from a manufacturers point of view (in comparison to Samsung, Xiaomi, LG, Huawei, Sony, etc.). However, if you want to spend some time and flash alternative ROMs yourself you are better off with Android due to the large modder community, but it also depends a bit on the device you bought.
My biggest issue on the other hand, is that if the manufacturers would also open source the drivers, they could be included in the Linux kernel and we would not have this discussions, because one could simply use an up-to-date kernel as you can with every PC.
1. UX: most of the time kernel updates don't affect the user experience. However, from time to time there are scheduler updates which can have positive effects.
2. Security: Being able to run the kernel with the latest security updates is evidently very important to have a system that is not vulnerable to newly discovered exploits.
3. Dependencies: As discussed already, some software components like the Android itself requires certain kernel features and therefore certain versions to let you run the latest versions of the software.
Btw. even LTS kernels are just supported for six years or so.
My biggest problem with the situation is, that 99% of the software is open source (Android incl. the Linux kernel) and just a few vendor-specific drivers make it very hard to upgrade the kernel and therefore the system.
backported 4.2, which includes some of the 4.3 changes as well. supports lineage. 4.1 is a version google supports till 2024, so I'm assuming 4.2/4.3 is going to be even later. So, you got a phone from 2011 that's going to run a modern kernel and latest android till after 2024.
> And to illustrate what I mean by ancient
yes. I would love to see an iphone from 2011 that's going to be running the latest ios and apple kernel after 2024.