Buttons and sliders are NOT cheap! Where did anyone get that impression?
Remember, you not only have to factor in the cost of the button or slider itself, but all the associated circuitry to interpret what it does and input the values (especially sliders) into the OS. You also have to factor in the increase in production cost to install all of the buttons and sliders, then the testing of them. The fewer the buttons, the cheaper the keyboard... If the original DX7 had been made with a button for every function, no-one could have afforded it!
And, let's not get carried away... Do you REALLY want to play something as button heavy as a Wersi or an MS..? Those things look impractical in the extreme for live use, with rows and rows of buttons, all doing something completely different, and one misstep in the middle of a live performance and who KNOWS what you are going to get..?
Maybe we could narrow things down...
What do YOU think is the two or three functions that currently you HAVE to use the screen for, that you would prefer to have on a button or slider? Prioritize your needs.
Here's mine. Octave transposition for Parts is on the screen (surrounded by other functions, although the 'button' area is quite big enough). I would prefer it on a couple of buttons. That's about IT, for me...
What's yours?