The big issue with major OS updates for me is that the security updates are bundled with the feature updates. That's why updating Android on my old phone stopped being feasable, when the increased performance requirements caused it to remain persistently hot and unresponsive.

Not just the security updates, but also things like APIs that prevent some applications from being compatible. Still, I have to wonder how much of a tradeoff would be possible when all the code is managed together, and the security and feature updates would be interwoven in the same repo. There is only so much manpower available to keep maintaining every version of each update.