Are We Sedevacantists Really Right?

At one of our meetings, a good friend of mine told us that his brother asked him a pointed question about sedevacantism. Needless to say, this brother frequents the apostate ex-lefebvrist society. So here comes the question.
If sedevacantism were true, how is it possible that the 400 priests (give or take) of the SSPX, with their seminary training, are not sedevacantists? Since there are not only a few of us, but most of us have no formal training in theology, how could we be right?
I responded that we wrote several articles answering this kind of argument, which I provided him. But I can give one more argument here that smashes the pro-neo-SSPX position more efficiently.
One knows that during the Anglican revolt, most bishops went into apostasy, with the exception of St. John Fisher. The rest of the dissenters were laypeople and some priests. Most priests went along with the revolt. The same is true about the Vatican 2 robber council. Those who went along were not only more numerous but also more erudite.
Therefore, I would respond with another question: how could the majority of the English clergy go along with the revolt while the dissenters were not only less numerous but less knowledgeable? Doesn’t it mean that the Anglican revolt was a good thing and the dissenters were wrong? And one can ask the same question about the robber council, not to mention the Arian crisis.