I actually don’t think the number of Cubase users depends mainly on the GUI.
Logic was designed specifically for macOS, so it’s naturally able to integrate very well with Apple’s ecosystem, whereas cross-platform applications have to adapt to multiple operating systems.
Personally, I don’t mind Cubase’s interface. I think there are other areas with much greater potential to improve the overall workflow.
For example, I’d much rather see the interface become more context-aware instead of simply being redesigned. Some people prefer rounded corners, others prefer a flatter or shinier look. That discussion will probably never end.
But imagine a context-aware interface instead. The Key Editor, for example, could automatically adapt to the loaded VST instrument by showing only its playable range, relevant articulations, or instrument-specific controls, while still allowing the user to display everything else whenever needed.
I think that kind of workflow improvement would have a much bigger impact than simply changing the overall appearance of the GUI.
I actually wrote a feature request that could be a starting point for this idea if anyone is interested.