This kind of surplus isn't sustainable, it'll reduce future tax income because of the economy shrinking. So the only way to force a future surplus is with even more austerity.

That's intentional, and seen by Milei as a good thing, just as it was for the libs of 90s Eastern Europe... and Thatcher and the like. Make the state dysfunctional to encourage the market to do it instead. It increases the profit rate for a few years (assuming people who rebel are beaten down, often literally) before market mechanisms mean the need to repeat the process.

The cruelty is the point, as is said here a lot. I predict a lot more Mileis coming to the west unless the proletariat finds itself wanting to act against the bourgeois state, capital, wage labor, etc. once again, instead of finding less unpleasant forms of capitalism.