Maxperson
Morkus from Orkus
Probably more than 5%, since that 5% number is the number of people who try to make it work without any support from WotC. If they supported it, more would try it.Well such a book is guaranteed to sell at least 5% as much as the core rulebooks, which means it will be at least twice as profitable as something like Journey Into the Radiant Citadel.
But I agree with you we are still unlikely to see it.