It seems to be an axiom in Judaism that no generation can be greater than previous generations have been (or, perhaps more correctly, no generation's leaders can be greater than any previous generation's leaders). No body today, it seems, could ever hope to be the equal of R. Moshe zt"l. Don't even bother trying to reach the greatness of the Chofetz Chaim zt"l or R. Chaim Ozer Grodzenski zt"l. The Vilna Gaon? Forget about it.
And, of course, if you go further back, this concept has been formalized into "law" in that no Acharon can argue on a Rishon (or a Gaon, or an Amora...)
But this is a concept that I have some difficulty understanding. The reason for this difficulty is technology.
I don't think anyone would argue that one of the greatest inventions of mankind (if not the greatest) was the printing press by Guttenberg. Before the printing press was invented, books were expensive (because they were all handwritten). The price of books was such that only the wealthy (or the clergy) could read because they were the only ones who owned books. 99% of the population was illiterate. The printing press, on the other hand, not only made book publishing less expensive, it also made printed material much more accessable to the general public - as now multiple copies of books could be run off at once, rather than having each copy handwritten. Furthermore, because the books were printed off of a press, there was much less of a danger of transcription errors, variant texts, etc.
Because of the printing press, knowledge in the world increased exponentially. Science as we know it would not have been possible without the invention of the printing press. It was one of the factors directly responsible for the Rennissance in Europe.
Now, with the printing press in place, it wasn't only secular works or Christian works that were published using this new technology - Jewish texts were published as well. It wasn't too long after the invention of the printing press that the first Shas was printed. Whereas, before, each Gemara had to be handwritten (making the times that the Gemara was burned in public all the more painful), now it could be reproduced much easier. More people could own their own copies. More people could own their own Chumashim, Mishnayos, Yad Chazakas, and so on.
Of course, as time went on and printing technology got better (we don't still use the same presses that Guttenberg used!), written works became more affordable, more commonplace and more accurate. Scholarship increased in just about all areas of human endeavor. You would think that this would include Torah as well, but that seems not to be the case. R. Moshe can't compare to the Vilna Gaon.
You can take the matter even further to today. Today, you have the Internet revolutionizing the way that people communicate with each other. It seems that it's come to the point where if a world leader sneezes, it's known about half a world away before the drops hit the floor (OK, I'm using a *bit* of hyperbole... but you get the idea). We can communicate with people instantly around the world. We can store materials in virtual libraries that anyone with an internet connection can access. I personally believe that when this time period is looked back upon in history, the birth of Internet will be more influential to world scholarship than Guttenberg's printing press was to world scholarship in his day and the century or two following it's invention.
Today, thanks to the internet, we literally have the opportunity to make the Torah globally available to anyone who wants it. Shas, Rishonim, etc. can all be put on line and made freely available. Much of it already is available on line. Translations of classical works abound in multiple languages. Shiurim can be recording on audio or video and placed on line for anyone to access. People can download and/or print out sheets to help them learn Daf Yomi. The possibilites to find ways to make the Torah available to people and to increase Torah scholarship have never been greater.
And yet, despite the potential for this explosion in Torah scholarship, it seems that no one can (as a rule) ever hope to equal someone from a previous generation. But why does it have to be this way? Why does each generation have to be part of some downward spiral to the end of time? Why can't a generation be greater than the one before it. I think that if there ever was the potential for this to happen, it has now arrived.
That being said, does Yeridas HaDoros *have* to happen? I know that there are some who look at the matter dogmatically and say "Yes, it has to happen..." But I don't want to accept that. I think that we can become better - we can strive to be better than our predecessors, especially with all the means that we have to disseminate Torah today.