By now, it would have been cheaper to put in a proper triple or quad redundant flight control system and have pilots retrain for the type than to ground the fleet and lose orders.

The bad decisions that led to this are well known. Southwest Air has a business model based on having only one type of aircraft, the Boeing 737. They insisted that the 737 Max behave exactly like the other 737 models, and not require that pilots be trained and rated for two different aircraft. So Boeing tried to make Southwest happy.

A deeper bad decision is that Boeing doesn't have a 737 replacement. The original 737 is from the 1970s. By now, Boeing should have had a new plane out in that size range. Airbus does. COMAC is coming out with one, if it works well. But Boeing guessed wrong on the sizes of aircraft airlines would want. They thought the future was widebody, not mailing tubes full of seats. The 767, 777, and 787 are all widebodies. So, of course, was the 747. The 757 was a great airplane for passengers and pilots that airlines didn't want because it cost more and used more fuel. So Boeing had a huge lineup of widebodies, and no good competitor for the Airbus narrow-body options.

Hence the 737 MAX, a 737 with engines too big for the airframe and a software kludge to compensate for that.