Originally Posted by
twistedenginestarter
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?
Because you don't KNOW what you'd be designing for. An aircraft design is already a massive exercise in compromise. If someone came along and said "I want all that lot, plus, if anything changes in the future, I want that too" the designers would just give up.
Bear in mind, however, that Boeing is already on the third iteration of the B737, so it's not as if they designed something that wasn't capable of being upgraded. But to imagine that someone sitting down to sketch the initial layout in the mid 1960s should have somehow tried to anticipate what might be wanted 50 years later ... they'd have been designing anchor points for the shielding for the airborne nuclear reactor that was going to be the power source!