I wonder for a long time now what Southwest is going to do when they will have to replace 737s finally. At some time at the future they will have depart from ol

I wonder for a long time now what Southwest is going to do when they will have to replace 737s finally. At some time at the future they will have depart from old and uneconomical (by future standards) design and at this instant they would have second incompatible plane operating in parallel and dramatic cost increase.