For M68 and onwards:
IsSecondaryUiMd() should always be true
ShowAllDialogsWithViewsToolkit() should always be true
And then we can remove all the code paths for either of these being false on Mac - i.e., all of the Cocoa secondary UI.
I'm calling this Fixed. There is ~25kloc left in c/b/ui/cocoa, but most of it is necessary platform integration stuff: AppleScript, various odds and ends of Cocoa UI that we want to keep in Cocoa (first run, task manager for now, renderer context menus), notifications, and share integration.