Anecdote has it that the 16-segment rule (including surface segments) came about with electronic ticketing, presumably because the record format was brilliantly () designed with that limitation.

The OneWorld Visit South America Fare also has a 16 segment limit, but at least a couple of years ago the AA ATW desk (which issues them) understood that to be a ticketing limitation, and had no trouble selling me more than 16 segments after some had been flown.

Not to suggest that anyone could find an agent who'd use that logic on xONEx - that chorus has been chanting '16' for too long now.