Who Will Stand the Test of Time?

Courant,nothing wrong with Foster and yes he does occupy a significant place in music as it relates to American cultural history.
However, comparing him to H. Schutz, perhaps second only to Bach as a composer of religious music, is like comparing a K-mart bike to a Mercedes S-500 .

Schubert wrote:

"...comparing him to H. Schutz, perhaps second only to Bach as a composer of religious music, is like comparing a K-mart bike to a Mercedes S-500"

No argument, and I agree completely. It appears I need to clarify...

I wasn't trying to defend Foster as much as criticize Brownsfan's using him as a counter-example choice to Schutz.

Essentially, my thought was that using Foster as a counter example was dicey in that, whether you like or can relate to his material, Foster is simply too important, even though he is a much weaker composer compared to Schutz. Foster will stand the test of time simply because of his place in musicology from a strictly historical perspective.

By analogy, F. Scott Fitzgerald during his lifetime earned just under $450,000 (about 10 Million adjusted for inflation). Source: http://theamericanscholar.org/living-on-500000-a-year/#.UWEqHldFIvo . In contrast Stephen King has sold 350 Million books and is worth $400 Million. Source: http://www.celebritynetworth.com/richest-celebrities/authors/stephen-king-net-worth/

My thought was that Brownsfan needed to use a counter example who was more of a musical equivalent of Stephen King who, if he's lucky, will be a curiosity in 400 years. Possibly someone like Elton John or Barry Manilow would have been better choices.

In contrast to "Barry Manilow", Foster was more the "Irving Berlin" or "John Lennon" of his day, and will be remembered.

Courant,
It does not appear that my statement and yours with respect to Foster's music are all that far apart.

"Most popular music describes a window in time. Once that time is past, the music becomes a relic that is at best a curiosity."

"Foster will stand the test of time simply because of his place in musicology from a strictly historical perspective. "

My perception is that the OP was not asking about music that would survive to be of historical or academic interest.

Ralph Vaughn Williams (along with many already mentioned).
Mapman - WHAT! no Rolling Stones?

Rok2id - in a future I'd prefer to experience, no one will recall Lady Gag Me or stuff by Tupak etc etc etc etc.

As an aside, and ignoring the commercial 'machine' that drives the pop industry (the more ephemeral exponents; Bieber, Gaga etc) consider the plight of classical music. In the 20th Century diatonic music went into a kind of decline: dodecaphonic, "music concrete", aleatoric music, etc all pointed towards the death of the kind of melodic and diatonic music that had served us for centuries.

Inevitable; the march of progress, one could say. In the 20th century composers like Rachmaninov could say things like "I'm old-fashioned because I write tunes". Well, history would bear out that this is what the average ear thrives on. Composers like Berio and Messiaen may never be popular: there is evidence that the average ear simply cannot comprehend and process this music. A composer like Schoenberg (and now we are talking about a music that is almost 100 tears old) will still not supplant the tunesmiths of prior generations. For the mass audience Schoenberg may still be close to un-listenable.

Could this signal the beginning of the end of an art form? If music can only look backwards in order to succeed then is atrophy inevitable? The number of live concerts that will place the "modern' composition at the end of the first half (so as the audience HAS to listen to it) is legion. La Traviata may always outsell Nixon In China - simply because it is more comprehendible. This has not happened before: no matter how much one might wish to think that 'new' music has always been the province of the avant-garde listeners this is very much the exception (the Rite of Spring is notable). Rather, new composers have tended to be immensely popular - in their time.

Perhaps humanity is at the end of one of its developments: in the visual arts too, one may make similar observations. I would surmise that Rubens would always be more 'popular' than Picasso for similar reasons. Time will tell.

Who will stand the test of time? Not Lady Gaga. And maybe not Webern, although I hugely hope I'm wrong about him. Diatonicists? Absolutely. But how? The answer maybe lies in a de-Westernization of the art.

I imagine the future will contain a more global music. In fact I think it will have to. Our thirst for new music is insatiable, but merely plodding on with a uniquely Western aesthetic maybe not fruitful. I am neither optimist nor pessimist: I heartily believe that music can be likened to a most powerful organism, and that it will do what it must to succeed and thrive. Diatonicism may be what we're stuck with, but there's a lot of diatonic music we haven't heard yet.

Long may it thrive!