Just as I was about to suggest commissioning Hans Werner Henze to work out a tuba setting of "Songs from the Arabian"....
Oh well, in for a penny, in for a pound. I'd like to venture that the tuba is, in fact, on the verge of obsolescence.
(on the verge of becoming obsolete, just to be clear) Or at least very quaint.
Take yourself outside of the role of tuba player/musician and try to think like Joe Sixpack.
The military will continue to use it in bands, there being no adequate substitute (maybe the sarrusophone will enjoy a rennaissance).
While it has a place in the orchestra, so does the contrabassoon. But I don't see a lot of major new works for contrabassoon either. I don't see orchestras on the ascendant, unfortunately.
What else? Some Dixieland groups still use it, as do a few polka bands (most do not). Wind bands use them, but how many professional wind bands can the average Joe on the street name?
Mostly, it's a school band instrument.
So what modern-day Beethoven is going to be motivated to compose a great work for it?