There’s a lot of critique about comparing sedevacantists to Protestants. I believe it stems from a misinterpretation unintentional or not of what is meant by the comparison. Is a sede as far gone as a current prot? Of course not. They have valid sacraments and just look at...

prot denominations now. But that modern Protestantism has fallen far does not mean that there is no similarity, especially when one looks at the reformers. It took a while until the anglicans lost the sacraments. Until then they were still wrong because they rejected the pope.

But of course rejection of the pope is not the only similarity and it’s not particularly close here. While the prots rejected the pope at first many of them did so because they claimed he’d acquired too much power that he should not have. Sedevacantism does not make this claim...

Instead it says that the pope has all his lawful powers but we just have a fake pope. Meanwhile rejection of the pope is one of the marks of all heresy. However, there is an important difference between sedevacantism and say Arianism, which doesn’t exist between sedes and prots:

acknowledgment of the magisterium. Most of the heresies of the church still relied on the bishops. One might disagree with them but it was still a matter that the bishops should eventually settle. Not so with Protestantism. In Protestantism the self becomes the magisterium.

Who judged that the church had fallen into corruption during the reformation? Not the magisterium, but individuals. Who judges now that the church has fallen into corruption? Not the magisterium, but individuals. And the same arguments are used by both.