Look at 1950s aircraft. That was the decade of really weird aircraft, as people figured out how jets were supposed to work. Supersonics. VTOL aircraft. The X-planes. Rocket-assisted takeoff. After that, more was known about what worked, and designs became more similar.

For more reading: https://en.wikipedia.org/wiki/Cambrian_explosion

During WW2 in the United States, you had all sorts of consumer goods companies reorganized to output a prodigious amount of military supplies. There were multiple companies making the same model of things, with fairly rigorous QA to ensure quality and uniformity.

For example, the BC-348 receiver, widely used in aircraft, was produced initially by RCA, and eventually "farmed out" to 3 other manufacturers.

More than 4 million M1903 Springfield Rifle were produced by the Smith-Corona typewriter company.

Here's a really good example, look at how the production of proximity fuzes, was distributed.[1]

The key thing is to have second sources for everything. Something the US military seems to have forgotten, or decided to ignore in their pursuit of gold-plated weapons systems that give the most kick-backs.

[1] https://usautoindustryworldwartwo.com/vtproximityfuze.htm

One design doesn't mean one factory. And it's not about one design anyway, just the thought of culling the less performing ones.