Note that just the above adds up to almost 10 years without a Pope. If we added in all of the other interregnums that were weeks or months long, we would certainly have many more years added to that.
It is not possible for an interregnum to be indefinitely extended and to say otherwise is an implicit denial of the perpetual necessity of the Papal power being transmitted to successors in the Church. There are some things only the Pope can do and if there is no Pope for too long, the Church will soon cease to bear her four marks, in particular that of Apostolicity, which is impossible. This consideration is sufficient to see the falsehood of sedevacantism.
You say it's not possible for an interregnum to be indefinitely extended. That is true - it cannot be indefinite; it must come to an end when the next Pope takes over. Since there have been several interregnums that have lasted years already, we can clearly see that didn't interfere with the 4 marks of the Church.
From the Catholic Encyclopedia on the article, "Election of the Popes":
"Between the death of Clement IV (1268) and the coronation of Gregory X (1272) an interregnum of nearly three years intervened. To prevent a repetition of so great a misfortune the pope in the Council of Lyons (1179) issued the Decree "Ubi periculum""
Notice the Church considered the three-year interregnum to be a "misfortune", but didn't say it was impossible. If it can be 3 years, it can be more years than that as well; the Church never specified a limit. So your argument here doesn't hold water.