Mike,
I agree with Dave that the 9000 was a great keyboard and its only failure was in the marketplace. For most of us, anything more than 61 keys doesn't improve our performaces, however, it usually adds a lot of weight to the board, while at the same time jacking up the price by a considerable margin. And, I suspect that you are right about a Tyros II having 61 keys as well.
As for their technical ability to create a 76 key, high-quality keyboard--they could do this with their eyes closed. However, I sincerely believe that they are merely following what the marketplace dictates and the bottom line sales of the products.
It's nice to have a fantasy--including keyboard fantasies,
Gary