Could you give an example of a good argument against sedevacantism?
I can think of an occasion on which a Traditional Catholic who accepts the Vatican II popes as true popes and who also knew his Bellarmine and Van Noort bested a sedevacantist in argument.
Some sedevacantists speak exclusively of heresy as that of which putative popes must be guilty if they are to be rejected as true popes. And then they go on to list vagaries that don't even approach the level of heresy. "He refused to wear the papal tiara. He dissed St Philomena. He looked at the scantily clad lady athletes."
When they might have checked out areas of "papal" vagary which did not excite them, which are not splashy, but which do approach heresy territory: speaking of Christ as someone Who had His "disillusionments" in this world during the Stations of the Cross, for example.
But no, I can't think of any good arguments against sedevacantism as its more savvy exponents present it. (And not that they agree on everything.)