"...those sopranos".

True, but that confirms my point, not the advisability of classical voice training.

What is most stunning about the sopranos is their achingly pure sound---achieved by straight tone, not vibrato.    Any vibrato they have is natural--not the operatic vibrato which is the subject of the thread. This is why every note they hit is crystal clear.

Many things can be learned in voice training--but the universally useful lessons can be learned by just singing many types of music in many types of groups---doing lots and lots of singing, especially a cappella, in groups, under good conductors.