True. There are people, even small people, that can shred in the low end of an upright bass, and those are typically 41-42" scale. The overall design of the instrument design is certainly a factor, as you mention. Technique is also a big part of the picture. It could be something as simple as adjusting the angle of the instrument in playing position (which relates to design), or something a little more involved like developing a left hand approach that involves more movement along the strings to compensate for the greater distance between frets in the low end. The bottom line is "the scale length is too long" may be true for some if they aren't willing to adjust their approach to fit the instrument. But I've seen enough videos of kids shredding full-size basses to know that it has to be possible to come up with technique that works just fine on these.