Boeing didn't panic at the NEO, their biggest customer(s) like Southwest wanted something ASAP that was competitive with the NEO. Airlines like EasyJet, Ryanair, and Southwest built their entire business on flying exactly one type. EasyJet with the A320 family and Ryanair + Southwest with 737. Southwest flies around 700 737s currently and retired their 737 Classics when it became clear that the FAA wouldn't sign off on pilots being rated on the Classic + NG + MAX at the same time (instead it Classic/NG or NG/MAX). You can build a profitable airline on a diverse fleet (look at Delta, they fly almost anything imaginable), but that's not where Southwest is now and it's an expensive road to diversify.

There's a nasty rumor that Boeing agreed to pay Southwest $1 million per plane if additional training was required. That's a lot of money just on the Southwest contract. If any other airlines (e.g. Ryanair) demanded similar that's a huge chunk of money.