Considering contraception was already not allowed when HV was promulgated and most priests soft pedal/ignore it, it’s doubtful HV is the reason behind the decline, and there is a vested interest in proclaiming the Novus Ordo a success - note the calls to ban the Tridentine Mass and the fretting about young Catholics that like it. These are not the attitudes of folks confident the Novus Ordo is vastly superior.