I wonder if it ever will stop?
I mean, all forms of music do eventually - they exhaust every direction available to them.
Like, ragtime ran out of steam, didn't it? Bluegrass likewise. If these exist at all, it's as a musical craft that people adopt as a hobby, something to master. A social activity. Appearances at local events or festivals. Bit like Early Music.
I mean, there's probably some horrible artist trying to combine ragtime or bluegrass with some other genre, or use a technology with it, but essentially, these things settled into static forms at a certain point.
Roots reggae likewise reached a point of formal stasis - you have bands playing that music all around the world, the original bands soldiering on, but also modern-day practitioners. But (admittedly I've done zero investigation into this, but judging by the odd whiff of it that's come my way over the years) newer artists have not added anything to it or extended it significantly.
They become traditional musics, to be upkept.
Perhaps rap, having a vital social function, that's the reason why it keeps on reinventing itself.
So something would have to supplant that social function, or society would have change so much, that the function would no longer need fulfilling.