$15K is pretty rarefied air. I'm guessing that most people that are authorized to spend that kind dough don't spend much time thinking about FF miles. So basically it's a case of diminishing returns. As the price goes higher, there is a smaller percentage who care about those miles and are actually incentivized by them.
When I was buying business class my fares to Europe were about $8000 RT and to China and Australia were $15,000 RT.
I sometimes spend $4-5K on coach roundtrips when I book last minute or have to book one ways to Europe.