I guess along the way, the
activity of designing and building the plane became more important than the
outcome / functionality of the plane itself.
You see it all the time in projects. People look busy and seem like they're building things, so some progress must be happening. Or is it?
Blink an eye, and in a couple years you unwittingly accumulated a platform that served to fulfill every random team's desire to load on requirements, systems, electronics, sensors ("go find out what people want!"), with very few people making the countervailing decision to trade off / cut things for an overall desired outcome ("what do they actually demonstrate -- not say -- they need?").
Well, they didn't have to pay for it, so there was no harm in giving their requirements. And if the actual willing payor (Congress) had little incentive (or technical chops) to be ruthless about costs or actual useful output, well then there's few checks on that happening. Until some top level general says, "why aren't these hugely expensive planes being used like we thought they would be?" Too late.
I guess it kept people employed in the meantime. How can you cut 10% of the workforce in Huntsville when the representative sits on the Armed Services committee? (I don't know that, but just for example...) Which, sometimes, is a national goal in itself for strategic purposes.
It might be good though, to have a more deliberate plan about these kinds of things, if that's the goal.