ian,

I cannot agree with You for one simple reason.

If it's "small piece of a small pie" that would mean that all current manufactures of 76-keys arrangers (Korg, Roland, Ketron) are doing something wrong, and they are doing it for a quite long time (decades), and some of them (like Ketron with SD-1 until SD-5 emerged) had only one, and only 76-keys proffesional arranger.

Well, if You know something about the market, You know that You can make wrong decision, but You cannot do it for a long time because You will vanish from the market, but it's not the case here.

Waiting for several decades to see if the 76-keys arranger market would be profitable is also out of the question, because it would be equal to waiting to see if the newly planted sequoia will grow 100 meter tall.

Here in my country (Croatia), and in the eastern neighboring countries (Bosna and Herzegovina, Serbia, Macedonia) almost everything that has letter "PSR" on it is considered a toy (with exceptions of 9000 Pro, and maybe S-900 lately). The most popular arranger amongst professionals (gigging OMB performers) was Korg Pa-80 not because of keybed, not because of sound, not because of interface, but simply because sampler and relatively affordable price.
These people would buy it even if it was 76-keys (if it would still be affordable).

I was quite surprised when I read that PSR-9000 Pro was unsuccessful for Yamaha because here it was one of the most desirable pro-keyboards (because of the included sampler, and quality of the keybed), and it still holds high reselling price.
Of course, it would be as successful as it was even if Yamaha decided to make it in 61-keys version only (like Tyros 1&2 are popular now).

So You can clearly see that the number of keys is not reason to buy or not to buy keyboard.