One is the Catholic Intuition of the Faithful, that comes from their Sensus Catholicus. And another is the Theological Proof left to Priests or Seminarians like me.
The Faithful rightly sense that an ongoing 61 year vacancy seems absurd, and that something like an alleged ongoing 600 year vacancy is absolutely heretical.
There are many Proofs, but the one I will give you is, From the difference between the state of the Church during the sede vacante and sede plena.
(1) During the time of sede vacante, no new Cardinal nor new diocesan Bishop or Ordinary can be appointed, no new Cleric can be incardinated into the Roman Church etc.
(2) Therefore, the first thing everyone will observe in an alleged long vacancy is (a) all Cardinals appointed by the "last Pope" as such will die. Then, (b) all Bishops appointed by the last Pope to office will die. Then, (c) all Roman Clergy in the Church will also die, and there is no Pope, per svism, to appoint more.
In your example of a 2 1/2 year vacancy it will not happen. The vast majority of the Cardinals, Ordinaries and Roman Clerics appointed or incardinated under the prior Pope will remain alive when he is elected. But in a 61 year vacancy (because Bishops are usually Consecrated around 35, as I noted to you before), it will.
(3) And certainly 2(b) and 2(c) are impossible. Therefore, it is likewise impossible that the state of sede vacante lasts longer than, loosely, a man's lifespan.
It is not possible that the state of svism lasts forever. The Dimonds are schismatics of the very worst sort; they won't listen to any correction from authority, and reject everyone. Consider it Divine Mercy if you're spared from falling into their errors, schisms and heresies. The Dimonds not only believe the heresy that sede vacante can last forever, but also the heresy that all sees can be vacant.
How anyone wants to hold heretical opinions like that, remain separate from the Church and the Hierarchy, and still hope to go to Heaven is beyond me. May God have Mercy on them and deliver them from their errors in due time. Don't fall for it if you want to save your soul and that of your loved ones is all that I can say.