The concerns mentioned seem valid enough to warrant discussion. Dialogue with traditionalists beats capitulation to the progressivists any time. The Catholic Church is the only institution that will entertain a dialogue with those to the “right” as well as those to the “left”. The left has borne the slogan “no enemies to the left” since the French Revolution and all secular institutions implicitly follow along, as well as many “religious” institutions.
Let the dialogue continue for as long as it takes and let the modernists splutter with apoplexy. At the end of the day, SSPX would do well to recognize papal authority, and the Vatican would do well to exercise its authority, mindful of its traditions and obligations.
They never denied papal authority as a principle despite occasional insubordination. They are not sedevacantists.
In my opinion, the insubordination, while a serious issue, does not outweigh the problems with false ecumenism and modernism that Bp. Fellay referred to.