It is rare for pop musicians to cross over into the classical world but when they do critics are unforgiving. McCartney got hammered when he tried it. I'm sure the complaints made are valid enough although I'm just as sure they are mainly remarks from snobbery.

It would be simple if there was a reliable assessment scale in music from zero to ten with rubbish being zero and excellence being ten. It would make things so much easier. But that's not how it is. Artworks are complex even those absent of sophistication. Merit is not only about years of endeavour, working the craft, mastering technique and learning convention. Ultimately, making a meaningful connection is the measure of any piece of art. Sometimes the work that does the business will come from some untutored source possibly ignorant of what had gone before, naive and spontaneous but still relevant. Connection is the primary arbiter not authority.

The popular music of the past fifty years is by definition contemporary and so it can't yet benefit from the kind of posterity that "serious" music enjoys which goes back hundreds of years and can look to a long history of appreciation and commendation for its validity. It may be a century from now before we know whether The Beatles are as good as Beethoven. If over the period people continue to value The Fabs then quite simply they will be.

Maligning the attempts of those in pop who adopt the classical form such as McCartney did may be justified but at this point it is hard to separate these criticisms from old fashioned snobbery.