Ok but the implicit assumption here is that a new design would not itself include all sorts of engineering trade offs between systems, maintenance, and pilot tr

Ok but the implicit assumption here is that a new design would not itself include all sorts of engineering trade offs between systems, maintenance, and pilot training, when in fact it would. The 787 was a ground-up new design and yet holds the distinction of the most recent previous instance of a full model grounding, over battery issues. Iterating on a proven airframe has a lot of advantages for aircraft design and it is common in the industry.