>Limiting themselves to a few expensive and stagnant vendors is a way to lose in the future.
You win wars by buying your equipment from the most capable suppliers. If that happens to be a centralized cabal of suppliers (this stuff is expensive, after all), then it is what it is. It's not the mission of the DoD to diversify the economy, its mission is to win wars as effectively as possible at any cost.
…and at the level of the DoD your goals include ensuring that there ARE as many capable suppliers as possible, using your commercial power, to increase your chances of “winning wars as effectively as possible”.
Wars are won on logistics; a corrupt, stagnant, or under-innovating market is a barrier to successful defence.
You aren't going to win wars by procuring from second and third rate suppliers just to keep the market diverse. You have to ask for proposals from across the market, but it's natural that suppliers will become centralized given how much money, time, and expertise is required.
The mission of keeping the economy healthy lies upon the Commerce and Justice Departments, whose missions are to manage the economy and keep industries within the confines of the law respectively.
Diversity, or "the condition of having or being composed of differing elements"[1], in this case a wide variety of suppliers.
I assume most of us are speaking English here.
[1]: https://www.merriam-webster.com/dictionary/diversity
>Limiting themselves to a few expensive and stagnant vendors is a way to lose in the future.
You win wars by buying your equipment from the most capable suppliers. If that happens to be a centralized cabal of suppliers (this stuff is expensive, after all), then it is what it is. It's not the mission of the DoD to diversify the economy, its mission is to win wars as effectively as possible at any cost.