

They aren’t trying to push the russians out. They’re trying to make staying impossible.
I think this is heavily under-stated. As long as russia is taking huge casualties without making significant progress, Ukraine is winning. All of occupied Ukraine (and more) is within striking range, so unless russia can completely neuter Ukraines ability to strike occupied territories (which is looking more unlikely by the day), it’s simply limited how long they can hold it. You can’t realistically annex a place as long as sticking your nose out of the bunker can get you killed.

I think there’s a slight misconception here (that is quite common). Let’s say you have an income of 100 Kronk, and a debt of 1000 Kronk. Further, let’s say that every year, your income is expected to increase by 5 %, and you spend 5 % of your income to pay down your debt. This year, you pay down 5 Kronk, next year you pay down 5.025 Kronk, etc. That means you can take out a loan of 5 Kronk this year, 5.025 Kronk next year, etc. in order to maintain a perpetual debt of 1000 Kronk, and a perpetual income after down-payments of 100 Kronk. (Adjust the numbers a tad to account for interest)
The point is: Even though you have perpetual debt, every debtor is being repaid on time. You’re basically just refreshing the debt. As long as people trust that you will be able to pay them back with interest, they have no issue giving you more loans - after all, they’re making bank off this.
This is overly simplified, but the idea is basically that as long as your income is increasing as fast or faster than your debt over time, debtors can trust that they’ll get their money back with interest, and you’ll be able to take out loans. This is basically why the US is able to keep taking up more loans, and have an ever-increasing national debt. Whether that trust is justified, and whether this is really a good, sustainable idea is a whole different question.
The issue russia is seeing now is that people don’t trust that they’ll be paid back with interest, at least not in a currency they care about. Basically, their income is stagnating (maybe even declining soon), while their debt has exploded. It’s more about the rate of change and prognosis for future development than the actual income/debt ratio today. When nobody wants to give them new loans, they can no longer “refresh” their old loans: Previously, they could just take out a new loan to pay down one that was close to defaulting if they were in a tight spot, they can’t do that when nobody wants to give them new loans. That means they need to spend even more of their income to pay down loans to keep them from defaulting (which would make the situation even worse for them).
So long story short, they’re pretty much spiralling at this point.