I know it's not ideal but realistically I think Behringer knows it's a niche product even among synth heads so making a 5 octave version isn't economically realistic.
As I'm sure you know, the common keyboard length of the old string machines was four octaves. Even that would have sufficed for so limited an instrument. But three? In the opinion of many of us, that's too short even for a mono synth.
Nah I disagree with that mono statement. I see no reason to have so many octaves when you can only play one note at a time. Zero real estate sense.
What about melodies that exceed three octaves? Mine do all the time. There are often no opportunities to hit an octave button. What then? Even another half-an-octave would be a major improvement, as with the Pro 2 or Minimoog. Whereas, my dear old Mono Evolver Keyboard was a constant problem for melodies, and I had to sell it for the big one.
Believe me, if three octaves sufficed, then I could happily afford one of the current mono synths. But they just won't do. I've owned six different three-octave mono synths, and each time I had problems with their size. That's a practical fact. Otherwise, there would have been a Sub 37 or Odyssey sitting in my music room long ago.
It's not without good reason that mono synths have been made over the years in all different shapes and sizes, so as to suit the needs of different types of synthesists.