Hmm, possible, I guess. However, the SSPX doesn’t have the power to ‘keep people from returning to Rome’. How would they accomplish this?
As with all priests and deacons (and ordained subdeacons), the SSPX members have taken vows of obedience to their bishop. Since the open dissent (which is not yet formal schism, but sure looks like material schism) of the SSPX, many have taken their vows to their SSPX bishops, specifically those ordained by them.
Returning to Rome separate from their bishop, and without their bishop’s release, they would be breaking a solemn vow, for those who were ordained by the SSPX at least, and for most of them formerly under Archbishop Lefebvre.
Further, the new priests ordained since the open dissent began may have been formed to expect not to be welcomed back, and we know they are taught that the Church has gone astray to at least some degree.
Further, they must, in order to come back, learn to say, and be willing to say, the Ordinary Form of the Roman Mass. Again, this is something they are taught is not appropriate.
There is thus psychological pressure to remain, even if Holy Mother Church would absolve them of that sin of Disobedience of their Bishop, and welcome them back.
They must overcome the errors they have been taught, and the perceptions of error in the Catholic Church in order to be functioning priests in the Catholic Church.
It’s not simple from their point of view… they must admit to
- having been wrong
- having been taught wrong
- having come to grips with those errors and seen that they are, in fact, errors.
That’s hard stuff to do.