The 737 was great design 20-30 years ago. Now it’s just not up to scratch. The NG was about the limit of what the design could take. The Max is not just a step too far, it’s a huge leap too far.
Boeing may not have the money, and they may say there are not the cost savings available in a new design at the moment. But the 737 has long outlived its design, and the more Boeing push that out dated design, the more we will see issues.