The general consensus amount Samsung users from my experience is that that they do these features better than Google. Samsung's multi-window support was better than eventually came to Android. Samsung's handling of app badges was WAY better (message counts!) and that went away with Oreo's terrible implementation (a dot indicating un-dismissed notifications).
My impression after upgrading to Oreo and getting more Google-designed features and less Samsung-designed features has been pretty disappointing. I lost a bunch of features that were extremely useful.
Unfortunately, as you said, reconciling the two code-bases means that Android updates are slow-to-never. The promise of open source and upstreaming patches for features doesn't happen with Android. Every manufacturer does many of the same enhancements themselves and then Google re-implements it a few years later poorly.
My impression after upgrading to Oreo and getting more Google-designed features and less Samsung-designed features has been pretty disappointing. I lost a bunch of features that were extremely useful.
Unfortunately, as you said, reconciling the two code-bases means that Android updates are slow-to-never. The promise of open source and upstreaming patches for features doesn't happen with Android. Every manufacturer does many of the same enhancements themselves and then Google re-implements it a few years later poorly.