This is interesting, and something I had never considered. My guess is that A.I. music will come to dominate the areas you mention, the royalty free arena of short films and youtube videos.
However I think there will always be room for human music, in fact I think it will always be the dominant form merely because music is about connections as well as the sound.
So there are many artists that are liked for who they are, and the particular way they express the emotion they're feeling via song.
Very, very interesting though, and I will chew this one over as it is a completely new concept to me.
There will always be a place for human created music, you are right, but for Hollywood studios with shrinking budgets is it really that far fetched that they insist on AI collaboration in order to keep 50% of writers share of royalties for themselves?