Religion as we know it will expire in...

I've been watching this very interesting documentary about the Secret Gospels and the whole history behind them. Don't worry, I won't go about analysing the fine theological points that rise from accepting that these documents are indeed what their name implies, and what could the reasons behind the Church not accepting them as genuine might be. I shall spare you that, and perhaps write about it in a future post. Or not.
What impressed me was a phrase by the narrator about the importance of something having to be old to have credibility. I'd better explain this.
Apparently, back in the 2nd (or maybe 1st) century A.D., a scholar/theologist started thinking about the apparent contradictions between the God in the Old testimony and that of the new testimony. His explanation was that, in fact, it was two different gods (or should I write "Gods"?), and the new one came to save the world from the old. His suggestion then to define the christian faith (as that was the argument back then, how to define a specific set of sources from which christians would get their common beliefs, a necessity in any religion and belief, I think) was to exclude the old testimony, with all the jewish influences and references to a vengeful God, and just keep bits and pieces of (what was to later on become) the new testimony. So far so good, a man's opinion is a man's opinion. The narrator then proceeds to say that, indeed, the God of the old testimony had posed certain problems for the early church in relation to how He appears through the words of Jesus Christ, so the thought of scrapping the old testimony altogether came about. He then proceeds to saying that this did not happen, as the link to the old testimony is necessary to create a link to the past (the very beginning of all, according to the old testimony in fact), which is necessary to give credibility, status and a sense of history to christianity, which would otherwise appear as a religion that started with Jesus. Which it is, really, but the point was that if it had all been foreseen and expected for thousands of years by prophets etc it would mean that people would treat it as a religion with a long past and, hence, quite worthy of respect, trust and, well, belief.
So my question is: why is that? I mean, why do old things appear to have a different gravity and importance? Why are, for example, old universities considered better than new ones? And I mean old, not just twenty years old. I do accept the fact that the more time a certain person or institution has been around for, the more experience they have, but certainly after 100 years, why should it matter? I am attending a university that is one of the oldest in the UK (oldest in Scotland, which doesn't necessarily mean that much, but there you have it), having been around for, oh, almost 600 years. Is it better than a university that's been around for 200 years?
Let's go back to the religious argument again. Old religions get more respect, yes? Well, actually not quite. The older a religion is, the more credibility it has, until it hits some limit, after which it becomes a thing of old and fades away. Did you say something? Let me refresh your memory. Christianity, old and all, right, but has been around really for a little more than 2000 years. What about the religions before that? What ever happened to the roman religion? The ancient greek religion? The celt gods? The viking gods? The aztech/inca gods? And I'm sure there's about a million and one different religions that escape my memory. Please also note that I am referring to western religions, as there lies my personal experience - although I'd be curious to see what's the story with eastern religions as well. Anyway, there seems to come a certain barrier previous to which things are not considered worthy of respect, but relics of the past.
So, old things are considered good up to a certain point, and then are discarded off. Think of a (rather) lame example, such is technology (before anyone points out to me that this is different to religion, as technology is a product of man, I'll quickly point out that religion - not necessarily the belief in the existence of divine beings, but definitely religion as a set of determined rules and practices- is exactly the same, a man-made construction), whereby things go from being new and swanky to old and better to antiques admired by some to heaps of worthless junk. Is that the fate of religions as well? Historically speaking, it would seem so. It seems possible that people in the future, believing in Llama Almighty or whatever will look back on christianity, budhism, hinduism etc and just wonder what all the fuss is about.
But is it really? Or has science covered mankind's need for change so effectively that the need for more steady pillars of continuity makes us turn to religion, thus leading to the prolonging of religion's life expectancy? And, if that is the case, what will the future hold for us? Has anyone ever thought that the reason why religions seem to sometimes struggle with the present times, or with the new conditions of the world, might just be because they weren't meant to last this long?
Is the end (of old religions) nigh indeed?