As mentioned, the 16 segment limit is due to eticket limits. etickets can only have 4 coupons of 4 segments.

Just after the IATA eticket "only" rule came in, I had an LONE4 with 16 flights. The last flight was booked as NRT-MEL. After ticket issue, QF caned the flight and I was rerouted NRT-SYD-MEL, making a 17 sector ticket. QF had to reissue it to paper.

Some agents can get around the rule by using the city code rather then the airport code (eg, NYC instead of JFK/LGA/EWR or TYO instead of HND/NRT).

There was a time before the current eticket IATA rules came into force where the "including surface segments" wasn't in the xONEx rules and it stated 16 flights.

The only thing stopping the use of a 5th coupon on an eticket (and thus a 17th+ segment) are the current IATA rules.