When it comes down to it, it is the distros (including Android) that decide which kernel versions they are going to use and for how long. Upstream doesn't alway

When it comes down to it, it is the distros (including Android) that decide which kernel versions they are going to use and for how long. Upstream doesn't always know this in advance, and only have limited influence in nudging distros to use certain kernels, since the big distros do their own patching and backporting anyway. There is no point for upstream to continue to support a kernel for 10 years if no one is actually using it 3 years later. So instead they give an initial support window, and extend it if the kernel becomes popular and no serious maintenance issues are discovered.