I think we can make the overall code of A-Frame shorter, hence better for loading for first time, not sure if this can affect negatively, if we make the core code of an "" accepts pointing to (referencing) any other entity to be used as its mixin.

if the issues concerning are related to the nested mixins (an entity referencing another entity through "mixin" comp that by itself is also referencing to another entity), we can add a condition to check if the referenced mixin is already an entity that referencing another entity and warn the user (who use inspector/editor) and developers (who directly code)..!