The goal is to lower seat-mile cost as much as possible. The 321 with 220+ seats are much cheaper than a 319 with only 138 seats. I believe all the 319s will be given back and nearly all the 320s will stay.
Indigo wants bigger planes with way more seats. The 319s just don't cut it, and I think we will see them all go away. I know that one of the 320s on property is being returned, but the rest should stay. If that holds true, we should see about 125 airplanes on property. I read about the 5 year time frame, but honestly I think it'll take longer than 5 years to get all the NEOs. I was thinking it would take more like 8 years for them all to be delivered. Perhaps the delivery schedule is faster than I think, but to get 101 new airplanes in 5 years seems much too fast to me.
Either way, we are moving to much higher seating capacity, and the 319s are going away in favor of 320s and 21s. Imagining having a 220 seat plane flown by an FO making 37 dollars an hour and a captain making less than 150... Sounds like a mngmt dream