If 61 note keyboards are the ONLY PSR's and Tyros's that Yamaha make, OF COURSE the majority of sales will reflect people's 'preference' for them, because THEY HAVE NO CHOICE!
The only way to find out what people's REAL needs are is to offer both sizes, and find out from that. Factor in the decreased cost of a 61, and naturally, you are going to find the majority want this size, but the cost of simply tooling the case larger to accommodate a 76, with no other changes made, is negligible. It's not like making a whole different keyboard.
So Yamaha ought to consider that those who MUST use 76, MUST use another manufacturer, probably something they would prefer not to happen. Surely the cost of tooling up for this would be MORE than offset by the large numbers (there are more than a few right here at SZ that have expressed this need - are we representative or not?) that want this product, and brand loyalty is prized when you are going for market dominance. To FORCE loyal Yamaha users away from the brand once their skills progress past simple LH chords RH solo techniques cannot be good for them...
An arranger is just a tool. What matters is what you build with it..!