What, exactly, is “classical” singing technique?
If you put 15 high-level classical singing teachers from universities or conservatories in a room and asked them what elements classical singing must have, the only things they would all agree upon would be that the inhalation had to be “low” and that “resonance” had to be enhanced. They would not agree on the details of anything else. If you asked about repertoire, they would not agree as to what way to approach repertoire or what kind of repertoire is best. In fact, they wouldn’t agree on anything except “breath support” and “resonance” as ideas.
That doesn’t stop the singing teachers of the world from talking about “classical training” as if it were a something. They also talk about the magic imparted by learning to sing foreign language art songs even if what you want to sing is “Stand By Your Man” or “I Love Rock and Roll”.
Classical singers still want to insist that all singing is the same and that you might “change the vowels” when you change styles. That is simply not true. If you want to sing all styles of music and sound like a classical singer while doing so, I suppose that all you have to do is change “the vowels” but if you want to change the rest of the sound, you had better understand registration, vocal quality and a bunch of other things like sliding pitches on purpose and making vocal noises for effect.
The attitude that vocal training makes all singing better is true if considered from a functional place but much vocal training is still not really functional (except by accident) and does not take into consideration that training has to go in the direction of the music being sung, even if you have to train the voice several different ways to do several different kinds of music. Classical sounds do not “elevate” other styles of music that don’t ask for them. Classical vocal production is not “better” than some other kind of vocal production as long as the sounds made are stylistically and musically viable and vocally healthy.
Understanding voice science does not automatically make you a good singer or a good teacher. You can understand voice science well enough to write articles or even books and still not sing well or teach someone else to sing well. You can have numerous graduate or doctoral degrees and still fail at both singing and teaching. What matters is the DOING. What matters is how well you sing, what criteria you have for your singing and whether or not you can convey that information to someone else clearly and easily.