This, he writes, might force "monopolistic" public transit
  systems to become more efficient by eliminating the
  possibility of route swapping--like by charging a flat
  rate per trip, as New York City’s Metropolitan Transit
  Authority does.

Or they could adopt a fee per minute of travel, or a fee for entering or exiting a turnstile, as regardless of the ticket swaps the number of travel minutes and turnstile turns remains constant.

Alternately, if they switched to an unmarked electronic ticket, so you couldn't tell if someone was offering you a valid or invalid ticket, the information asymmetry could cause bad tickets to drive out the good.