It isn't as simple as "make a new design with a shorter lifespan" either. These systems are intended to work the first time every time, and every change introduces numerous second-order consequences. You swapped the batteries? Great the balance of the missile now changed and we have to re-calculate the flight dynamics. Remove some shielding and conformal coating? Now the thermal properties have changed on the control boards and the welds are cracking due to different heat propagation. We certainly could make them cheap and dirty, but their reliability and consistency would suffer. The last thing you want is to shoot a missile and piss off the guy on the receiving end, who isn't dead but is now very motivated to get revenge.