It depends how you interpret “Can’t be improved”

Most manufacture have to release updates to their newly released arrangers as there are either bugs, or sounds and styles are just not up to scratch.

Yamaha very seldom have to bring out anything other then minor updates, (And those are rare) as they get it right first time.

Large or major updates will necessitate a new board, (That’s just progress) but until this is released the owners just enjoy what they have got, as it suits their current needs and does what it says on the tin.

Just check the various arranger forums, and you will find that Yamaha owners are the most satisfied out there.

How do they do this?
They do their research, and provide what the vast majority of people want. (That’s why amongst other things you will not find a Yamaha 76 note arranger or one with a heavy weight)