I donâ€™t think the original question was about designing a piano. Isnâ€™t there a easy answer to the original poster? Over 99.9% of all music does not need more than 88 keys, and we can discuss only a handful.
If we can think of only a couple of pianos that have more than 88 keys, that should be enough to satisfy the OP. Most concert pianist play on pianos that have 88 keys both at home and in concert
Well, I *DID* address the musical question first.
Building at costs that are acceptable to wider audiences COULD encourage greater acceptance, use and composition for the greater range.
As long as >88 is only available at Bosendorfer and Stuart prices very few of us will even try to play the limited repertoire and little more will be written. Chicken/egg egg/chicken.
Not that "build it and they will come" always works, but there are SO MANY examples of the market not knowing that they wanted something until it was offered.
Market research answers hind sight questions, etc.
"At home" I would like the range of 96 or even 108 for under $5K, preferably under $2K.
No good reason, a "like", a "want", not a "need to have: so I can play a particular set of pieces.