If I tell myself that there is no Pope then I can continue to do whatever.
Do not take the following as an endorsement of what the sedevacantists (SVs) think.
SVs do not “put themselves in the Pope’s place”. They believe that there is a period for as long as 61 years (depending on what SVs you talk to) when the Church has been without a pope. They do not try to do or say anything a pope would, it is just as though we are in an interregnum between popes, but a very, very long one. They do establish chapels, ordain priests, and consecrate bishops to ensure that everything that their faithful need is available to them, as they see the situation. They make the presupposition that not only is the Holy See vacant, but that the bishops and priests have fallen into heresy
en masse. (There are some variations on the theme, such as sedeprivationists,
materialiter-formaliter SVs, proponents of the “Siri thesis”, and even those who say Benedict XVI is still the Pope — the latter are not SVs now, but they will have to reconsider things when Benedict dies.) There are some who object to one SV bishop’s episcopal lineage, and some who object to that of another SV bishop, and so on. In short, it’s a mixed bag and they’re “all across the boards”. It is, to say the least, interesting.