If the gates of Hell did prevail (had prevailed), it would mean that there was no one (but especially among the clergy, representing that Church) who believed and practiced the True Faith any longer.  But this has never been true in Church history -- never, and it is not true today.
 
"Hell not prevailing"  does not mean that those of the True Faith will be in the majority, or that the Pope will be close to an ideal Pope (or even a good leader).  That's not what the word "prevail" means.  

The mere fact that the restoration is very slowly but very steadily growing means that Hell has not prevailed.  The battle is just working toward intensity because the forces of Hell are also getting stronger at the same time.

The Book of Daniel says "The eternal sacrifice will be ended." This means the eternal sacrifice to God, the Holy Mass (the sole means by which grace enters the world) will be ended. And yet, if only one faithful Catholic remains, the gates of hell will not have prevailed.

So, what needs to happen for the gates of hell to prevail?

In the end, Satan would need to emerge the Victor - which can never happen.

I like this snip from a sermon given by Fr. Wathen that I think applies to this subject:

"....We have reason to celebrate at all times because as you know, even though we commemorate the sorrowful event of Our Lord’s Passion and death, and even though we live in a very sad situation, we realize that our sorrow is not that of complete tragedy. We might even use the expression, “The Divine Comedy”, of which we are players, because everything that happens is under the Divine Control. And even though there is much sin and there is real tragedy, the only real tragedy and the only irreparable tragedy of course being the loss of a soul, and this actually happens.

But as far as creation is concerned and our part in it, we do not really recognize any tragedy because we anticipate salvation. We have no reason to disbelieve that Almighty God does not intend to save us. Not that we are not able to lose our own souls, but we recognize that we are serious about being saved. We intend to be and we have reason to rejoice that Almighty God has shown us such mercy.

Among all the mysteries that we live amongst, is that of the fact that God saves those whom He wills. And yet those who are lost are lost because they will. No one is saved against his will and no one is damned against his will. At the same time almighty God has known from all eternity who would be His whom he would succeed in saving. And all the jubilation that the Church expresses in its many Masses and in its Office, is over the fact that those whom God regards as His elect will be saved.

Furthermore, that no matter how much tragedy with which history is strewn, Christ moves towards His glorious triumph. With His resurrection was the announcement that He would have his victory, when he emerged from the tomb He proved that there was no force, no power greater than He, and he proved that if He was invincible, then that which He would establish is also invincible, namely His Church.

It really does not matter therefore that throughout history the Church suffer terrific blows, that it at times – and these times almost have always prevailed – that the Church suffer it’s terrible embarrassments, it’s setbacks.

Despite all this, despite all appearances and despite whatever losses, Christ is triumphing in the Church and He is proving His power, His invincibility and He is succeeding in doing what He came to the world to achieve, and God the Father is fulfilling the purposes of His creation.

If it were not so, He would never have created anything to begin with. If it could be, that Almighty God could set in motion anything out of which He could not draw whatever He wished, then He would never had done anything like that and He indeed would not be infinite in the first place.

We have it in our power to participate, or we have it in our choice to be turned away, it is strictly within our choice, and whatever grace is necessary is within our grasp...."