While I am the last person to have an unnatural desire to apply Marxist historicism to everything I can get my hands on, it's worth noting that Classical music is, in much of its history, a matter of social distinction. You go to Classical music concerts if you're rich and you can pay for it. These boundaries are largely destroyed in the advent of commercial recording, but even now they still exist. If you want the real taste of Classical music, the live performance, the aspect of the connoisseur, you must go rub shoulders with the city councilmen and the local music theory faculty at a live performance. Maybe your dentist, your family doctor, your French teacher. Chances of seeing the plumber there? Adjuster? Not really that high. It's a bit like golf, but worse.
Is this a bad thing? Not necessarily. But it is worth noting that it can be fallacious when people formulate bombastic apologiai for Classical music, trying to win "back" the younger generation and the majority of people to the aesthetically superior form of music. The problem is, Classical music has never had a wide audience and, quite possibly due to commercial availability, never wider than today. It isn't a matter of getting Classical music back to a position of wide-spread appeal, since it may not have had it in the first place.
On the other hand, not all Classical music is like this. It doesn't seem likely that you're going to hear Brahms' 2nd piano concerto if you're a 19th century Dickensesque inn keeper. It does seem considerably more likely that you hear a Bach cantata if you're an 18th century inn keeper. Bach's music was music that was heard in church and, while, at its more corrupt moments, the Church's liturgy has been a matter of class, music of the church is music that is heard by all the congregation. And, in early Lutheran Protestantism more than anywhere else, perhaps, the music is informed (but not dictated) by the congregants' needs and their level of musical education. These early Protestant churches love congregation-lead music.
I've talked before about the tune that most of us sing O Sacred Head Now Wounded to—it was apparently a German love song. Both his predecessors and Bach himself took this tune and set it many different ways. (And, so did a few composers who came after him, most famously Brahms.) This is an example of the composers of High music lifting folk and popular tunes out of the culture and implementing them in a religious context. I think it would be a little unrealistic simply to attribute that to some already-existing quality in the folk song that merited its use in worship. Bach et al were, to some degree, trying to teach and edify by using something that would be, to most of the congregation, familiar.
Bach is an excellent example, but he's not the only example. As I've mentioned before, Josquin and Dufay do this with the famous tune L'homme arme, just barely leaving the tune recognizable but ubiquitous in their Missae L'homme arme. There's even a rumor that the tune Pange Lingua, to which is set St. Thomas' famous poem, was in fact a Roman marching song.
But does that mean that modern Church music composers should take a "melody" (coughcoughcough) from Coldplay or Regina Spektor and use it as the basis of a Kyrie? Yyyeah, no. That's dubious, to say the least. Not that I have anything against Coldplay or Regina Spektor (that would be appropriate in this post), but the issue is one of apples and oranges. Bach and Josquin were working on generations upon generations of people who had built up a Christo-centric culture. The music that came out of that culture was certainly not sinless and certainly not totally consistent with a Christian worldview. But the music didn't clash with Christian doctrine. Today, we have a totally different field to play on. Inevitably, a song (regardless of the ethos of its authors) that hits the top of the commercial music list is going to clash with the catechesis of Christian worship. That doesn't mean it's not going to work 100% of the time, but it does mean that there are many more issues to weigh in the balance and many more insidious cultural connotations to deal with. Bach and Josquin had a level playing field—they weren't in a battle with the majority of the culture. We are.
When the music of a culture is defined by the music of its Church (I don't think it ever isn't), when, in short, at the center of the public square is the music of worship, the effects on folk and pop music will be drastic. When this happens in Christian Churches, and we all come to hear, worship with, love, and understand the High music of the church, the gap between the aesthetically richest 10% and the aesthetically poorest 10% will be narrowed. But, as usual, it doesn't work to regulate that. It has to happen organically, over the course of generations and academic years and, uh, counterpoint classes. But then we won't be far from the sort of culture that writes quodlibets for a pastime. Everyone will be edified, musically, by being immersed in music they have, over time, come to understand. The give-and-take between the music of the Church and the music of the culture will be such as was not really possible for the majority of the history of Classical music.