Actually like Hanyuu says, in mania, things goes extremely wrong.
Because currently 7K gets a high bonus in "star difficulty", a common 7K Normal would just become something like H or even I.
See my previous map
https://osu.ppy.sh/s/92190 for an example, in this map, the 4K Normal GD is labeled as Insane, but it actually is easy enough that it is a easier-than-common 4key normal. While this is ranked before the adjustment of osu!mania stars, the extra bonus would make things worse. (And you see, the Hard is labeled I)
And I don't think that to label a map with its calculated difficulty is something absolutely true. When mappers have their own thought, and the song objectively is a potential hard map source (e.g. High BPM, DnB, 1/4+1/8 beats without any main instrumental line), the "easy" difficulty may be just as hard as something "hard" in other songs. Yeah, other music games does that, some "easy" labeled songs are extremely hard that it's already harder than most of "hard" labeled songs. Not to say that is a good idea (in mapping style), but when such maps are considered to be ranked, the diff spread saying Insane, Super Insane, Another, Black Another and Lunatic is far worser than Easy, Normal, Hard, Insane, Extra.
So support +1