I'm inclined to agree with your conclusion, overall.
In this case it's worth noting that the issue is ameliorated by the fact that ScopedTypeVariables can only change the meaning of code that already uses other extensions, since it only affects type variables explicitly listed in a forall, and by default you can't have explicit foralls.
In this case it's worth noting that the issue is ameliorated by the fact that ScopedTypeVariables can only change the meaning of code that already uses other extensions, since it only affects type variables explicitly listed in a forall, and by default you can't have explicit foralls.