The problem was hardware manufacturers ie processor etc would not support newer versions of the OS and kernals. Thats why they have come up with project treble with that you would be able to install newer versions of android long after they themselves stop giving updates.
Treble doesn't help anything because OEMs are the ones still pushing updates, and only devices released with Oreo are oblieged to be compliant with Treble.
Which is why even in 2018 most new devices have been released with 7, upgradable to 8.
But I am glad OEMs keep ignoring Google, as they finally added update clasues on their licensing contracts.