Speaking in very naive terms, "glueing" the Psr 9000 pro keyboard mechanics (that many people here have praised) in a Tyros design, surely is not a big deal for the hardware designers of Yamaha.
Actually IMHO it is nothing, probably something like "plugging it in". No major design changes.
Makes you wonder why they won't do such a thing. Tyros is too much money and too much keyboard for me to buy, but I realise that 76 keys mean world to many pros.
Send a picture of yourself, playing a competitor's keyboard in a pro situation, with the quote "I would buy Tyros in an instant, if it had 76 keys, so think again!" to a Yamaha exec.
Maybe that will work. At least you will get a laugh.
Maybe that will work.