Sancrosanctum Concilium 116

How strongly did Vatican II (!) say Gregorian chant needed to be preserved at the Mass?  (More importantly, why did no one listen?)  Fr. John Zuhlsdorf weighs in here, and says:

The Council Fathers in Sacrosanctum Concilium go on to talk about the use of other kinds of music and they provide a welcome flexibility.  But none of those other provisions eliminates or supersede or mitigate what SC 116 says.  In other words, we shouldn’t justify the use of Gregorian chant.  The Church has done that for us.  We have to justify the use of something other than Gregorian chant.  (Emphasis mine)