I think the Vatican’s preference would be for Williamson to sign on and then be sent to the SSPX home for retired priests, or an SSPX-affiliated monastery somewhere.
The problem with not having him on board is that he is a bishop, meaning he can consecrate more bishops among those who might follow him.
On the other hand, having him on board and giving him faculties brings its own problems.
Alas, as the late Dom Gerard Calvet, OSB warned Archbishop Lefebvre in 1988 when Fr. Williamson’s name was added to the list at the last minute: “I support you in consecrating the original three candidates, but not Williamson. That man has never been a Catholic. If you consecrate him, he will destroy the SSPX from within and lead it into schism within 10 years of death.”
Dom Calvet, protector and restorer of Benedictine traditionalism, Ora pro nobis!