There's a lot to digest in your last post, accusonic - but let me at least start with one or two quick points - maybe these will start to grease the wheels of this discussion a little bit.
I'd like to clarify my original remark regarding percussionists playing and writing only for each other. I was thinking not so much about education (although the insularity of academic culture plays a role in this), but more about why, after almost 100 years of explosive development, percussion music is not as accepted or widely known by the general public as, say, string quartet or brass quintet music.
Two quick thoughts (or maybe not-so-quick thoughts) came to mind:
1) Maybe this is just based on my own experience, and isn't accurate, but I'll throw it out there anyway, but...it seems to me that in many ways, the type of percussion ensembles we're discussing here are mostly linked to academia - witness The Percussion Group (Cincinnati College-Conservatory of Music), Bergamo's Repercussion ensemble (the school where many of those players teach escapes me at the moment)...when I think of perc. ensembles *not* directly affiliated with a university of some sort, I think of Nexus, and maybe Zivcovic's Trio - and I'm sure there are others. The fact that so many of these ensembles are connected to institutions of higher education means, IMHO, that the "insularity of academic culture" can also show its influence on these ensembles' approaches to their programming of pieces, to their marketing of the ensemble (or lack thereof), etc.
I also suspect that the academically-oriented nature of the Percussive Arts Society has something to do with it - exactly what, I'm not sure, I can't quite put my finger on it - but that organization (of which I am a member, and I have been for years) has always struck me as being much more of an educational society, linked inextricably with academia, rather than a performance-oriented organization, although both aspects of the art are certainly represented there. Maybe that's the way it has to be for PAS to function, but that just reinforces the influence of academia on contemporary percussion, including percussion ensemble literature.
2) Musically, percussion ensemble music has indeed progressed dramatically in the past century - but in terms of percussion ensembles (or percussion soloists) working their way into the "mainstream" of the classical world is concerned, let me ask: what percentage of works performed by orchestras, are works which vary from the basic "please the blue-haired old ladies" pieces that seem to fill the lion's share of orchestral programs (at least in the U.S.)? I think there's a general bias against programming newer literature - percussion-based or otherwise - and percussion ensemble music (and percussion soloists) tend to get swept aside as a result.
If the orchestras and/or contractors don't think there's a market for new(er) music, that's almost as powerful (more powerful?) than the audience not wanting it - and it can certainly circumvent whatever interest the audience may have in wanting to see more percussion music.
I think that a market can be created for percussion ensemble music, but it has to start at the grass roots level - library concerts, children's concerts, and the like - mixing education with entertainment. I don't mean we should program a medley of Britney Spears tunes arranged for marimba band, but rather selecting wisely from the existing percussion ensemble literature, or commissioning new works for percussion ensemble, etc.
There are marimbists out there touring as recitalists - not a lucrative career, but it's something that would not really have been considered a viable option in this country before Leigh Stevens (among others) started carving out his niche. I think that percussion ensembles could do something similar, but not by simply replicating the programs they played in college.
There is, IMHO, an audience out there for new, creative music - but they have to be cultivated, which is an art in itself.