I'm a bit confused where OP is leading here but I don't think war is any good for any economy (besides whatever tech might be invented in the process). It'll take years to repay those loans and to build back to the same prosperity as the pre-war version of the country.
As for old money, don't know why you mentioned them but they are more likely than us to survive wars. Even if their assets are destroyed or taken away, you can bet they still got enough hidden to rebuild again.
Basically the idea is that war (and many other things) makes fiat world get a bit worse, and fiat devalues there, and if you are smart enough to take a loan out of it, put it on dollar, and then cash it back out, you could make a lot of profit. Consider this, you are a business person, you have a chance to get a 1 million dollar loan in Russia, but in rubles and not in dollars.
You get about what, 77 million ruble back when it was decent? So, they tell you that you need to pay back like 85 or 90 million ruble, even 100 million ruble when you want to pay that loan back.
But you do not pay it, you just hold it in dollars, now that the war happened, it is 110+ or so in the market, so you just made yourself another 30+ million ruble without doing absolutely anything. Obviously timing that would be hard, but it shows you how war and devalued fiat could help you with money.