Interesting discussion, but so far I did not see the most important reason to choose for 76 keys instead of 61.
We are talking arranger keyboards!

So if brought back to the basic funtion of the arranger keyboard, left hand acc and right hand melodyline, fils etc...

It all depends how are you playing your left hand acc!

one finger or full chords!

To get most out of it you use full chords.

So 76 keys at least, since you need 2,5 octaves to bring in all the nuances.

So is it correct to say that the 61keys players don't play full chords, but one finger only...?

I am aware that this last sentence will steam up some but hey who cares...

Fred