Would love to hear your take on Vatican II, since that led to a dramatic shift in what is and isn't Catholic Orthodoxy. Although I'm not Catholic, a few that I've discussed theology with are pretty adamant that the Church was hijacked in 1958 and is no longer the real Church.
Although I'm aware that sedevanctists are definitely in the minority, you have to admit that claiming to be the one and only true way, the O.G. form of Christianity doesn't exactly fit well with how different the Catholic Church is before and since Vatican II. The gradual shift since then towards acceptance of things like homosexuality, abortion etc sort of lends credence to that idea, from an outsiders perspective..
When I've asked this before, most mainstream Catholics try to handwave it away, but the ones I know that are most steeped in Church history and their beliefs feel quite differently.