Complain as you may, don't forget that there are sometimes good reasons for modifying both vowels and consonants when singing.
Microphones are, compared with our ears, over sensitive to plosive and sibilant consonants. Softening "S" in the direction of "SH" and "T" towards "D", and trying to avoid "P and B" when singing close mic are examples of good microphone technique- but they will tend to make a Briton sound American.
With vowels it's more complicated.
"Opera singers especially subscribe to a certain artificial pronunciation."
This makes musical and acoustic sense.
The formants that give vowels their distinctive sounds are sets of harmonic frequencies that overlay the fundamental pitch of the note. At some pitches, these frequencies do not sit happily together. Basic musicality will lead to a tendency to shift vowels towards those that sound better at that pitch.
If they all sang "mid atlantic"* it would be fine.
*that is the phrase used in the UK, whatever it means in the US.

It's not such a clear cut case of mindless imitation as you think, and it would often sound worse if it was not done.
Have you ever wondered why some of the best sounding choirs are regional?
If you had both Welsh, French and Yorkshire people, singing perfectly in tune, in English, in their own accents, there would be some horrible clashes on certain sounds- actually on most sustained vowels.
Cheers
Dave