It's not like more than 1% of android phones that have shipped to date will actually get an android update to Oreo anyway so why not leave them behind and update the kernel?
New phones need new drivers which must support the new kernel, period. I don't see what the big deal is. You aren't getting Oreo on your old ass Samsung Galaxy S2 anyway.
GP's point was about OEM support for newer versions of Android, though; I can't imagine that third-party ROMs on old devices are even slightly a factor in Google's hesitation to upgrade to newer kernel versions
New phones need new drivers which must support the new kernel, period. I don't see what the big deal is. You aren't getting Oreo on your old ass Samsung Galaxy S2 anyway.