I think you can have peak musical forms.  There will always be another great pop song, but the low-hanging fruit were picked in the late forties to the early seventies (I think.)  With classical, there was continuing development up to--okay, up to 4'33".

My optimistic thought is that new ways of proposing the world to ourselves and each other naturally genetate new musical forms.  Someone said the other day that if you took current television programming, plonked a plasma screen in the middle of a nineteenth century street and switched it on...no one would watch--it'd just be noise to them, too quick, unintelligible.  Maybe a very slow moving image would capture their attention...

Moving that thought into the future, new musical forms are presumably developing now, but we may not be able to hear them--

How would Prokofiev have sounded to the ears of a troubador back in the (?) thirteenth century...but there'd be a couple of musicians who might think, "ah, hmmm, okay, hmmm.  Ah!  Okay!"

(I'm working on the theory that not every brain is optimised for its current cultural traditions--hence some brains are more adapted to picking up ideas from other cultures--whether they be past or present...)

Heh!  Not sure if that answers the question!

Don't fight forces, use them R. Buckminster Fuller.