Yep, same can be said of manufacturing capability in general. If you don't have a manufacturing base then inevitably when there's a war you may find yourself unable to build defense components.

Let's imagine a hypothetical symmetrical war between two modern countries. One can disable the other's satellites and maintain their own fleet. The other can't get access to any third parties' satellites.

You aren't going to send your steel navy out when one side can see you from space and you can't, almost regardless of the numbers. Your big army of steel tanks is useless against a bunch of drones directed by distribute satellites.

would it be situational re economies, though?

i.e. if you have a lot of financial resources and can buy 1000 ships cheaply for the cost of propping up your steel industry to build 100 ships, should you buy 1000 ships (or the steel needed to build them quickly) to instead?

Steel on its own is inert and useless, you need to retain the entire downstream manufacturing ecosystem that consumes it. Like cars for example, producing steel but letting all your vehicle manufacturers sell off to foreign owners just so you can import BYDs doesn't do you any good.

You can make this same argument about a great number of things. Why is steel any more critical than food or vaccines or the like? Indeed we got caught short of vaccines recently, and had some nontrivial consideration of running a military op against NATO member over it.