In a way, most Protestant Christians are sede. Most point to the ancient unified Christian Church as having been authoritative and “pure”, at least up to the 4th century, when the NT canon was more or less defined.
They don’t exactly pinpoint full authority in the early papacy but they presume whoever was leader was guided or respectful of “purity” of doctrine, say in 300 AD, and that there came a point, say 400 AD, when the Church leader promoted false or man made doctrine. They might argue the chair of the early leader, passively handing on biblical teaching, was usurped.
Some “traditional” type Protestants, such as Anglicans and Lutherans, seem to assume the Catholic leadership was reliable up till a later year, since they accept early development of doctrine (in effect, overseen by popes) but not development under later popes, closer to their own beginnings.
So, in a way, sedevacantism is later-day Protestantism.