This really isn't a very intelligent question. The term "sedevacantism" is of relatively recent origin. I've read various claims that the term was coined as recently as the early 1970s. But claiming that because the term didn't exist in the past the very concept is therefore impossible is using the same logic that many Protestants and Modernists use to claim that Matrimony wasn't really a sacrament in the early Church because the word "sacrament" wasn't applied to Matrimony by the Church Fathers for several generations.
While I don't have the specific reference available (perhaps a more learned member can provide it) theologians have generally held that, while during the Great Western Schism there was indeed a true pope, it would not have been impossible for the interregnum to have lasted for the entire period of the Schism. Thus, while the specific term "sedevacantism" was not used, as it had not yet been coined, the concept of a long interregnum, had already been discussed and deemed a theological possibility.
Personally, I do not like the term "sedevacantist". Clearly, if there are "sedevacantists" there must be "sedeplenists" (i.e., the seat is full). Instead, we should use the word "Catholic". If one holds all the Truths of the Catholic Faith, he should be considered a Catholic whether or not he knows who the current pope is. If one does not hold all the Truths of the Catholic Faith, he should be considered a heretic, apostate, or pagan (as appropriate) even if he is convinced beyond a shadow of a doubt who the current pope is.
Before the instant transmission of news became available during the last couple of hundred years, does anyone think people went to hell because they didn't know who was pope at any given moment?