I think it would take either ordaining female priestesses or celebrating sodomite marriages.
I have several conservative friends and I have directly asked them if they would consider the Conciliarist modern Vatican II anti-popes as such if they would start ordaining female priest would that be "the sign" and for most of them it would be an overwhelming no. No doubt there are a few who sympathize with the SSPX and it would be these folks that would actually jump ship, to them there is a limit. For most conservatives it is useless to debate with them because if they can't answer the question of what it would take for them to be convinced it will end up being a circular argument. It is like trying to argue with a Ku klux klanner why hating on blacks for no reason whatsoever is silly, but some people really are irrational.
I think celebrating sodomite marriages, might seriously disturb them temporarily making them cross the threshold to the SSPX position but nothing more. There are some that are culpably blind and could careless what happens. To them its just about the easiness of the new religion, quite frankly. It is too hard being a practising Catholic in the modern world.
You mean I can't cohabitate with my girlfriend?
You mean I can't use contraceptives or NFP? Get "Catholic" divorce through the Conciliar Church (invalid)...
You mean I have to abstain from meat every Friday?
You mean I have to attend mass on Sundays and Holy days of obligation on their actual days?
You mean I can't pray with my x or y family member who is an apostate/heretic/false religion?
You mean I can't read heretical non approved books? WOW! Quit oppressing me!
You mean that as a woman I have to be submissive to my husband (if I am married)? You are a mysogonist!
You mean that I have to obey the state so long as they don't ask you to sin?
You mean drugs are mortally sinful to use?
You mean that I am not the saint that I have imagined myself to be?
You are a party pooper and that religion is too hard to follow.
My response would be think not about what you "are giving up", but rather the treasures that you are accuмulating in heaven. It is easy to serve the Lord most especially because He has given us the graces to fulfil our duty. Do anything but sin, which gives you the true freedom to do quite a bit. Our soul was meant to be with God, and if we want true happiness we must desire to be with Him for all eternity.
Do not be surprised in the amount of people that will never wake up, the problem is at the root. Their principles are completely twisted and that is why their theological conclusions will always completely fail in the most epic manner.