Moreover, from the musicians perspective, improvising is extremely fun. And it's what most people would do, if they have some time on their own with a piano.
So training in improvisation should have a significant demand from the musicians themselves.
Only 199 people have signed up for Prof. Mortensen's classical improvisation course. I wouldn't call it "in demand" among classical piano musicians themselves.
Sadly, "classical" and "improvisation" became almost a contradiction in terms during the 20th century, and this seems to have stunted the creativity of so many classical musicians in the last couple of generations.
However, let's say you are alone in a dark room, with a piano, and no-one else can hear you, and you are in a whimsical or creative mood. Do you want to play other peoples' music, which expresses them - or would you like to create something yourself, that expresses your own soul, and the current moment in time?
Being able to improvise is perhaps analogous to flying, or at least cycling without training wheels. Intuitively most people would love to do this.
Improvising puts you into a creative trance, and it is possibly the most enjoyable thing you can do with your instrument. The problem is that it requires a lot of practice (and musical knowledge) to get good at it.
My "free improvisations" are not very great and I doubt anyone would ever pay to listen to them, as they involve a lot of noodling and wrong notes. Nonetheless, to my ears, they are very exciting, as they express my original ideas, and my emotion in the moment. I don't think there is any substitute for that.