I guess I just feel like if something "is diegetic"--even "is diegetical"--then we should all agree it is so.
It should not be, "Well, to me, that's diegetical. But to Alex, it's not-diegetical. And to Pat, it's debatably-diegetical. And to Sam, it's diegetical on odd-numbered days of months, and not-diegetcal on even-numbered days, except February 29th, where it is diegetical only if it happens before noon, and not if it happens after noon."
That is, when we talk about diegetic music, that's...a thing everyone agrees on. Either the character in the film is experiencing that music, or they are not. That's...pretty much essential to what "diegetic" means, in-context. There isn't, and can't be, any subjectivity in whether the music is diegetic. It's possible to be factually mistaken, e.g. to have missed how the decrescendo in the music corresponds precisely to the moment where the character turns down the volume on their MP3 player (or whatever)--but it's not possible for Alex to assert that they feel the music is diegetic, while Pat asserts that it feels non-diegetic to them, and both are correct.
That seems utterly essential to what "diegetic" means. Why should that be abandoned?