it s an agreement only ,
anytime, they can cancel the agreement. it s not the first time that airlines canceled their order.
advantage: they order planes in advance when the 320 is fully booked for the next 3-4 years.
disadvantage: Indigo loose money if they cancel their order with airbus, and no one know when the big crisis in India will come.
to finish: dont belive there will be a pilot shortage in india.