It's incredible some people think the sede sect, or the sede Church if you will, is supposedly the Roman Catholic Church. Let's call the idea that it is the s-RCC. The mainstream RCC we will call m-RCC. Then, (1) Either the s-RCC or the m-RCC is the t-RCC (true RCC), but (2) it can be shown that the s-RCC is not the t-RCC, ergo or therefore, it clearly follows that (3) the m-RCC is the true RCC.
We could lack at all 4 Marks, but for brevity, we'll take just 2.
(1) Catholicity or Universality: Is the sede sect geographically universal? To ask the question is to answer it. It is nowhere near to it. Mal 1:11 requires a Universal Church with a Universal Sacrifice, and the m-RCC has it, with 1.4 BN Catholics and 400,000+ Priests in every nation, but the s-RCC does not. And if you think this argument is novel, it is based on the Gospel itself, which says the Gospel will be preached, and thus the Church present, in all nations, until the end of the world, and is the precise refutation Pope Bl. Pius IX used against the Old Catholics in Etsi Multa against their false claims: "23. These unhappy men undermine the foundations of religion, overturn all its marks and properties, and invent so many foul errors, or rather, draw forth from the ancient store of heretics and gather them together and publish them. Yet
they do not blush to call themselves Catholics and Old Catholics, while in their doctrine, novelty, and number they show themselves in no way to be either old or Catholic ... Christ Himself is asked; He says ‘and
this gospel will be preached in the whole world, in testimony to all nations, and then will come the end.’ Therefore the Church will be among all nations until the end of the world. Let heretics perish as they are, and let them find that they become what they are not.”[8]" From: https://www.papalencyclicals.net/pius09/p9etsimu.htm
In the below article, Non-Catholic Christian writer Philip Jenkins shows there will soon be more Catholics in Africa alone than there were in the entire world in 1950! "
By 2050, a conservative estimate suggests there should be at least 1.6 billion Catholics ... If current trends continue, as they show every sign of doing, then by the 2040s there will be some 460 million African Catholics. Incredibly, that number would be greater than the total world population of Catholics as it stood in 1950."From: https://catholicherald.co.uk/catholicisms-incredible-growth-story/2. Apostolicity: The CE says this: "
"Apostolicity is the mark by which the Church of today is recognized as identical with the Church founded by Jesus Christ upon the Apostles. It is of great importance because it is the surest indication of the true Church of Christ, it is most easily examined, and it virtually contains the other three marks, namely, Unity, Sanctity, and Catholicity." https://www.newadvent.org/cathen/01648b.htmSo Apostolicity guarantees the other 3 marks are present. Where is Apostolicity found today, in diocesan Bishops appointed by the Popes, or with the sedes? The CE continues:
"Billuart (III, 306) concludes his remarks on Apostolicity in the words of St. Jerome: "We must abide in that Church, which was founded by the Apostles, and endures to this day.: Mazella (De Relig. et Eccl., 359), after speaking of Apostolic succession as an uninterrupted substitution of persons in the place of the Apostles, insists upon the necessity of jurisdiction or authoritative transmission, thus excluding the hypothesis that a new mission could ever be originated by anyone in the place of the mission bestowed by Christ and transmitted in the manner described." The words are very clear. Apostolicity continues in the dioceses, by Papal Appointments conferring the Apostolic Mission, just as it did before Vat. II"
And as for unity, needless to say the SSPV, CMRI, Dolan, Sanborn etc who in some cases don't even recognize each others valid or licit orders do not have unity. But Catholicity and Apostolicity are sufficient to demonstrate it.
"He who has ears, let him hear".