Plug compatibility
OK - perhaps I'm asking a different question. Why don't plane companies design their planes to take new engines, new instrumentation etc thoughout the lifespan of the product?
It must have been obvious that, when the A320 and the 737NG were on the drawing boards, new more efficient engines would become available as technology moved on. Similarly with electronics, wings etc.
Every now and again you need to make a fresh start, but it's odd Boeing can't easily respond to the NEO. Their order book is healthy enough to suggest the enormous cost and disruption of a new plane is not necessary.