Now, I realize that most people do not want to think on the weekend. However, I have the backing, let us say, of Joseph Cardinal Ratzinger's God and the World. In the section on the Church, the Pope, then Cardinal, states that although the Holy spirit ensures that the Church will remain until the end of time, She, Our Holy Mother Church, may not be everywhere. Let us look at the passage:
There is not doubt that the Church gets tired. The Church can even become tired throughout an entire cultural area and even come to ruin there. The Apocalypse warns,, in the letters to the seven churches, which point to an example for the future: Watch out, or I shall take the lamps away. It is in fact the case that not only the flourishing Church in Asia, which was once the vibrant heart of the Church, but also the Church in North Africa vanished entirely beneath the tempests of Islam. Of course, the force applied by the conquerors, the repression of whole classes of society, played their part here; we don't want to make an oversimplified complaint against a tired Church. But still, this can happen.
The promise of Christ--"Behold, I am with you always, until the end of the world"--does not mean, in addition, that each and every diocese has the assurance that it will endure forever. It does mean, on the other hand, that the Church will be, until the return of the Lord, His special sphere of action, His organism, His Body, and His vine.
Now, I can think of dioceses which could disappear, not from conquest from without, but conquest from within. One only has to think of those dioceses with the liberal bishops who do not stop disobedient Catholic politicians from receiving Communion, or bishops who allow priests to be involved in "gay Masses", or dioceses where the vast majority of the women contracept.
God is Just and Merciful.