I believe that the use of recycled designs is one of the major guarantees for aircraft safety! There is nothing wrong with a gradual development process, where only small modifications are done from one revision to the next. This is a bit like continuous integration principles in software development. Building on former experience as much as possible is an important factor of quality assurance (and also efficient in many other aspects).
I understand your point, though: small modifications can sum up to a point, where they do not blend smoothly in the base design anymore - as is the case with the larger engines of the 737 8 max. At some point, designers have to refactor big.
What went (criminally) wrong with the 737 8 max, in my eyes, is the attempt to treat a large redesign as a small modification, to save (certification) time and money. It went horribly wrong, causing loss of lives and trust in the company.
And obviously Boeing is not able to understand the consequences of the loss of trust. Last week I thought, the worst thing that Boeing could do now, was to offer quick fixes (like hot patches for the software), as this would proof their incapacity to embrace the scale of the problem. But this is exactly what happened.