Even if we were to take your word that Yamaha T2 and PSR 900 (61 keys) outsells Roland G70 and E 60 (76 keys) it is a flawed argument to make that 76 keys do not sell.
Look we all know that it is not the keybed, keyfeel or number of keys that sell the Yamaha arrangers it is sounds, styles and OS. And apparently, a number of people prefer Yamaha to Roland, Korg and Ketron.
The reason why Yamaha refuses to listen to its customers is because of the unsuccessful of 9000 pro. But the problem with the 9000 was not the 76 keys but the glitches in the OS that it was oversized and over weight. Their refusal to make a 76 key arranger has nothing to do about Yamaha thinking a 76 key arranger would not sell as much as the 61 key arranger.
If the Roland G70 had the same size and weight as the T2 and had 61 keys, are you telling me that Roland would be the same as a T2 in terms of sales. If when the T2 was unveiled with the innovative SA voices and styles, it was 76 keys in stead of 61 keys are you telling me that the T2 would not have sold as much as it did?
Its all about sounds styles and OS.
As Yamaha has proven with the PSR 900 and T2, undesirable looks of the keyboard, a poorly built body and keyfeel and keybed does not decrease sales as long as the sounds, styles and OS are the best.
So it begs the question, what are they afraid of with 76 keys?