"If people keep broadening the term 'bluegrass' it will become meaningless."

This has been an unsolvable issue from the start of bluegrass's existence. Bluegrass is a hybrid music. Ralph Stanley doesn't like being described as bluegrass - he calls it old-time or mountain music. I can see why, too: if you get too fundamentalist about what is or isn't bluegrass you'd end up listening to a very monotonous procession of variations of I-IV-I-V chord progressions with entirely standardized sequences of fiddle, mandolin and banjo solos. Mostly in the key of G major.