In T92540#3177828, @Esanders wrote:

Or we could try and actually support this, by manually going up the inheritance tree and mixing everything in top to bottom.

I can't find where I mentioned this but we discussed this before. The feature was declined for two reasons:

• A mixin is based on the idea of copying over properties at run-time without inheritance. Breaking that inheritance by flattening it seems non-deterministic. The mixin would initially set up live inheritance that is than broken at an arbitrary point in the future when it is applied.
• The concept of composing mixins is already trivially achieved by simply using mixins instead of inheritance.

class MixinFoo
 ...

class MixinFooBar
  inherit Foo // !

class QuuxFobo
  inherit Quux
  mixin MixinFooBar // mixes MixinFoobar but not MixinFoo

class MixinFoo
 ...

class MixinFooBar
  mixin Foo // !

class QuuxFobo
  inherit Quux
  mixin MixinFooBar

class MixinFoo
 ...

class MixinBar
 ...

class MixinFooBar
  mixin Foo // !
  mixin Bar // !

class QuuxFobo
  inherit Quux
  mixin MixinFooBar