Ignoring the speculative nature of the original post, I find it ironic that, when you get right down to it, the only real reason such a papal "refutation" of "feeneyites" would have any "umph" is precisely because subjection to the papacy, adherence to the Catholic faith and reception of the Church's sacraments are essential for salvation.
To argue that the papacy, the Church, her doctrine and her sacraments are, at the end of the day, not necessary for entry into heaven is ultimately self-defeating. Why would anyone take an institution seriously which spends its time condemning people who claim it to be essential for salvation?
Besides, it's not as if we have more serious issues to deal with. Let's spend our time attacking our fellow trads and causing even more infighting and fratricide.
That'll help the Church. For sure.
After all, isn't it obvious that the greatest threat to the salvation of souls, the propagation of the Faith, and to the survival of civilization are those pesky "feeneyites"?