There is no way Ukraine is going to pay its war debt. Their currency and economy has only been propped by western capital, once the flow is reduced their currency is going to collapse. The reason the Hrivna is being propped is that Ukrainians would move to the Russian-held parts which will be functional, especially pensioners and retirees.
Blackrock and co. pushed for loans to Ukraine with the understanding that they would be repaid with Ukrainian arable land, minerals, mining rights and other assets. This is going to be a big problem politically speaking, as Ukrainians are not going to like having their assets stripped on top of a depleted male population and a destroyed economy.
They will try to appropriate seized Russian billions to remedy their situation, but the Russians will require a return of their assets as a basic condition for an end to the war, where they have had the upper hand and their advantage is only going to get wider.
So bottom line, we have to eat Ukrainian debt, it's the least we could do knowing that we have pushed Ukraine to fight a war they cannot win.