It's an interesting question.

Could we save ourselves the cost of a military, and put that money to better use, without actually endangering the lives of our citizens?

I would suggest, certainly in the case of the USA, that the answer is "yes". The USA's military budget is larger than most of the rest of world's combined. Yet the USA is not in any danger of being invaded. This massive, massive, military force is entirely about projecting force elsewhere, diplomacy by other means. It's not "defence".

The USA has huge social welfare problems. Hundreds of thousands of homeless people, for example. If the USA chose to, it could redirect even a small part of the military budget to building social housing, and (imho) the world would be a better place.

So while I agree that having a military for defence (and purely for defence) is probably not a bad thing, that's not the purpose that most countries have a military for. Hegseth aside, most anglosphere countries have a Department of Defence that has never had to defend anything [0], but has been at war for most of the last 50 years. I think this is bad.

[0] Falklands war is a little bit dubious; technically a defence of a UK protectorate. But colonialism, etc.