The first point is that much of this music is simplistic (a very different concept to simple) and often trite as well.
However - with what are we comparing it?
When I say that I'm comparing it with pieces such as:
Jean Langlais - Messe Solennelle (here the Gloria)
http://www.youtube.com/watch?v=BMnDOd4Hv1s&feature=relatedTomas Luis da Victoria - Requiem (what I hope to have for my own funeral)
http://www.youtube.com/watch?v=HKo1wHtbNgYRobert Parsons - Ave Maria
http://www.youtube.com/watch?v=f2WrDvI847gCharles Gounod - Messe Solennelle de Sainte Cecile (here the Sanctus)
http://www.youtube.com/watch?v=vA2jzQq4bUM&feature=fvstI could go on all day.
This, clearly, isn't what a typical parish will be singing on a Sunday morning...never was and never will be.
If we compare what went before the Council with what is around now, the typical Catholic then (at least in Britain and the U.S. and probably in Australia as well due to the Irish influence) would have had low Mass with hymns...and the sort of hymns that would have been sung were "Soul of my Saviour", "Hail Queen of Heaven" and so forth, similarly musically trite.
My view is that one shouldn't despise simple music merely for being simple - it doesn't get much simpler musically than plainchant which was specifically called for by the Council, it can also be sung congregationally as witnessed by the Salve Regina from Lourdes
http://www.youtube.com/watch?v=9Zto0Q4EmTE