its not one but 76 reasons.
the only reason i can think of for upgrading a Tyros 4 to a Tyros 5 is the 76 keys..
Upgrading older Tyros models and PSR models is allways nice, because from a 3 or older is actually a pretty big step upwards as it is atleast 2 incremental steps. Actually buying a new arranger every 6 years seems the most efficient for most.
And then there is those people that allways needed or wanted 76 keys and never found that with yamaha despite them actually loving the yamaha sound.
tyros 5 is nothing but an incremental grade up, as all tyros models where. but if you compare it to my T1 from ten years ago, its a huge step.
This is how Musicians tend to see it.....
Over in europe its a different thing, selling T5 mainly depends on how many free goodies require a T5 over a T4. If there are enough audio style packs and new T5 only sounds, or just T5 only ipad apps... people will buy...
Dealers are allways giving the official yamaha packs and stuff for free to their customers, just to persuade them to buy a new one. If someone figures out how to create their own audio styles or convert those from Audya, then people will absolutely buy T5, selling a new arranger overhere in europe becomes more reliant on data and software then on the actuall hardware. Because all these oldies buying these instrumement are style and sound collectors and not great musicians.
With this knowledge its obvious that T5 will sell again.