Yes I have wondered what they will do about the 319 replacement as well. They do have some new ones. So it seems likely they will continue to operate it. But like you said the ability to acquire more/replacements is not there.
Personally I despise the 737 from a pax perspective. Even in February they are hot in the back. The seat bracing forces you to sit sideways and there is little in the way of under seat storage.
Vasu the great will have to start making difficult choices soon however. By years end we will be down ~65 airplanes between the 80's going away and undelivered/grounded MAX's. Schedules will be cut and staffing forecasts reduced. Unfortunately Delta is taking on aircraft and adding to their route network as us, United, and SWA have to pare back.