If Paul had granted Catholics permission to contracept, then you might be correct that they did everything they could have to undermine traditionalism, but since he didn't grant permission tto Catholics to contracept, then he didn't do everything he could have to undermine the Church.
However, if you want to place blame on the post-conciliar popes for the way people disobey or ignore the Church's teachings, then you would be correct, for it is Humanae Vitae that is most responsible, and therefore the two men responsible for it.
Neither Paul VI nor JPII was ever interested in changing moral doctrine. Both from time to time made declarations supportive of traditional Catholic morality--Paul VI with Humanae Vitae, JPII with countless declarations against abortion and homosexuality.
But neither was fully supportive of the traditional faith itself. Both Paul VI and JPII knew exactly who their opponents were. Inevitably the men who were isolated supported the old theology and the established dogmas of faith, particularly Transubstantiation and Propitiatory Sacrifice.
Not daring to deny the doctrines of faith directly, however, both popes deliberately created facts on the ground which would inevitably undermine the old faith everywhere and which carried the virus of modernism into the bosom of the Church.
The one concocted a new vehicle for transmitting a protestantizing, watered-down faith--the New Mass. The other pushed an unprecedented ecumenism that bordered on indifferentism and syncretism. Both deliberately suppressed whatever dogmas were uniquely Catholic.
There's no mystery to any of this. Paul is on record as approving Blondel, the father of modernist philosophy. John Paul gave red hats to modernists like deLubac--who openly denied dogma--and Lehman and Kasper who were publicly heretical. Both pontiffs persecuted Lefebvre who clung to the ancient faith.
Many believe it was not possible that in such a short span of years such a systemic collapse of the old faith should have been unintended by the Holy See. I concur with this analysis. It certainly seems intended. That is to say, it seems to have been planned all along at the very top.