Thatīs a commom published strategy at EZY.
The 737 are not that old, the 737-300 are all phased out.
Somebody got to fly the Boeings. The size of the 737 fleet is in an economic range.
If you have two different fleets working economic, you always have a door open two both manufactures for what ever might happen.
So, itīs a way to go.