The kernel started moving faster, but also the kernel for these devices are orphans. They can never be updated properly because the source code for the kernel modules is not available. The way the phones get updated in ROMs is by back porting changes from the present back to the old orphan. It requires someone super familiar with both kernels to do so. Eventually it becomes untenable. The whole scheme of Android is centered around this source code/orphan kernel scheme. Everything is setup so that hardware manufacturers never have to add their source to the mainline kernel meaning you can never own the device. There is not a single phone or mobile device that you can completely own and running on mainline with available source code. The pixel is all about the TPM chip.
I think I did at one point but it did not work for me. That was a long time ago though.