I consider the wildly mixed UIs not a symptom of compatibility but a symptom of the succession of hopeless attempts to be more than that. The incomplete new GUI generations are surely not caused by compatibility burdens affecting the kernel team.
No but they're a symptom of the 'all work on the same product' situation - if they were split, the people wanting to update the UI etc would work on the new version alongside the kernel team for the new version.
