One point that nobody has made yet is that the reason why opera singers strongly trill their 'R's and emphasise their consonants is because they're singing without mikes to a large theatre. If they didn't do that, you'd only hear the vowels and not the consonants. The same applied to music hall artists.

Modern popular singers whether Bing Crosby or Stevie Wonder have never had to do that, so a whole style of singing has grown up which just sounds completely different to anything before the 20th century, and fans of popular music just expect their singers to sound the way they do.

Of course, folk song was never meant to be sung in a theatre.