Sez you.
The 63-year (and counting!) Interregnum (time period between Popes) has got to be the Achilles Heel, if not the fatal flaw, of Sedevacantism.
That's better than claiming that the Holy Catholic Church could produce a Mass that's harmful to faith and displeasing to God, and could teach error, if not heresy, and lead souls to hell.
As far as a lengthy interregnum:
Theologian Fr. Edmund O'Reilly:
The great schism of the West suggests to me a reflection which I take the liberty of expressing here. If this schism had not occurred, the hypothesis of such a thing happening would appear to many chimerical. They would say it could not be; God would not permit the Church to come into so unhappy a situation. Heresies might spring up and spread and last painfully long, through the fault and to the perdition of their authors and abettors, to the great distress too of the faithful, increased by actual persecution in many places where the heretics were dominant. But that the true Church should remain between thirty and forty years without a thoroughly ascertained Head, and representative of Christ on earth, this would not be. Yet it has been; and we have no guarantee that it will not be again, though we may fervently hope otherwise. What I would infer is, that we must not be too ready to pronounce on what God may permit. We know with absolute certainty that He will fulfil His promises; not allow anything to occur at variance with them; that He will sustain His Church and enable her to triumph over all enemies and difficulties; that He will give to each of the faithful those graces which are needed for each one’s service of Him and attainment of salvation, as He did during the great schism we have been considering, and in all the sufferings and trials which the Church has passed through from the beginning. We may also trust He will do a great deal more than what He has bound Himself to by His promises. We may look forward with a cheering probability to exemption for the future from some of the troubles and misfortunes that have befallen in the past. But we, or our successors in future generations of Christians, shall perhaps see stranger evils than have yet been experienced, even before the immediate approach of that great winding up of all things on earth that will precede the day of judgment. I am not setting up for a prophet, nor pretending to see unhappy wonders, of which I have no knowledge whatever. All I mean to convey is that contingencies regarding the Church, not excluded by the Divine promises, cannot be regarded as practically impossible, just because they would be terrible and distressing in a very high degree.
...and, writing about the Great Western Schism:
We may here stop to inquire what is to be said of the position, at that time, of the three claimants, and their rights with regard to the Papacy. In the first place, there was all through, from the death of Gregory XI in 1378, a Pope — with the exception, of course, of the intervals between deaths and elections to fill up the vacancies thereby created. There was, I say, at every given time a Pope, really invested with the dignity of Vicar of Christ and Head of the Church, whatever opinions might exist among many as to his genuineness; not that an interregnum covering the whole period would have been impossible or inconsistent with the promises of Christ, for this is by no means manifest, but that, as a matter of fact, there was not such an interregnum.
Beside that, the see has only been vacant since the death of Cardinal Siri (Pope Gregory XVII) in 1989.
Then, of course, there's the sedeprivationist position, which also upholds the integrity of the Magisterium and Universal Discipline ... and overall Holiness of the Church.
There's nothing inherently incompatible with the promises of Christ to have a lengthy interregnum.
Meanwhile, nothing is more contrary to Our Lord's promises than a Church actively leading souls to hell and whose public worship of God displeases Him.