You're wrong on part and right on part.
WRONG about the parts not being made anymore. That didn't matter. What mattered was that COTS solutions were NOT ALLOWED until the past 15-20 years.
So, you're RIGHT that "so everything had to be custom designed and manufactured. Which with a limited run is rather expensive. Even for a fucking toilet lid." But, that was because the government regulations MADE them do that. In many cases they could have found a COTS solution to the problem but were restricted from implementing it.
They did that for tooling in many cases too. Need a wrench? Nope, can't buy it from Sears. You have to make it. Oh, you only need 6 of them and the tooling to make the wrench costs $24K, well, looks like you've got some $4k wrenches.
And while 3D printing is good for some things, my firsthand experience with it in NUMEROUS applications from large simulators to small UAS it's not the end all be all solution to a lot of issues. Don't get me wrong, 3D printing CAN be really useful. We print a lot of smaller things at my current job. Some are good, some are meh - the carbon fiber ones tend to be better, but more expensive.
I spent 30 years in Aerospace and Defense before moving to a small UAS research center so I've seen this shit for a LONG time.