Other countries do quite well military-wise, without having domestic aerospace industries. They just buy the planes from other countries that do have them.
Maybe not feasible on a scale such as the US military, but still.
The problem is that this gives other countries a big stick to beat you with, just by refusing to sell you more aircraft or spare parts or whatever. Iran is a case in point. Under the shah, Iran was a US ally, and purchased US aircraft. After the revolution, the US shifted its favour to Iraq and refused to sell spare parts and supplies for the aircraft, severely hampering the Iranian air force. This might not have been such a problem, but Iran was at war with Iraq at the time.
This sort of thing is why most countries are keen to manufacture as much of their military equipment domestically as possible.
Who don't? Governments certainly do. Food is commonly subsidised so that domestic production is maintained. Oil is commodity number one in governments exerting influence to control it. I'm not so sure on computer manufacturing.
Or if not subsidized, then subject to enough import restrictions that domestic producers survive. I'm a pretty hard-core free trader, but in this case I think some restrictions to ensure a local industry make sense. You shouldn't have to starve or freeze just because other countries close the border. Security comes before prosperity or you're partying on the Titanic.
What do you think why, if a German comoany such as Infineon or Aixtron plans to sell parts of their business to the Chinese, the US President gets a veto?
Because you do apply the same logic — it has to be produced by us or our closest allies — to all these industries.
Maybe not feasible on a scale such as the US military, but still.