When Pope John Paul first allowed the celebration of what we now call the "Extraordinary Form", he required both the group requesting the EF and the priest celebrating it to affirm explicitly their acceptance of the legitimacy and authority of the Second Vatican Council and the liceity and validity of what we now call the "Ordinary Form."  John Paul required, as a condition for the permission,
". . .that it be made publicly clear beyond all ambiguity that such priests and their respective faithful in no way share the positions of those who call in question the legitimacy and doctrinal exactitude of the Roman Missal promulgated by Pope Paul VI in 1970." (see: https://www.ewtn.com/library/CURIA/CDW62IND.HTM)
I think Pope Benedict made a serious error in pastoral judgment when he lifted that requirement.  I think Pope Francis thinks so, too, though it is not at all clear what he can do about it, or what he is willing to do about it, particularly while Benedict is still alive.