> you don't really need to worry much about supporting legacy products years after selling them to customers

You really, really do. Missiles are expensive, and stay in inventories for a very long time, and they need to be made compatible with every update to every platform that can make use of them. That wouldn't be so bad, but then you also need to prove that they work with all those platforms. This is hard.

> they are not expected to be functioning after just one use.

Missiles are only fired once, but that doesn't mean they are used once. The typical "use" of an aircraft carried missile is that it is attached to a plane, powered up, and then the plane does a sortie and lands, and then the missile is removed and maintained. There is a lot of maintenance that is done to the missile daily.