In a nutshell, the economy comes down to this: we have available labor time and natural resources with which we can produce some stuff and distribute it. Unemployment now means lost production now that can never be recovered in the future.
Unemployment now just means we produce less than possible now. The sacrifice does not enable us to produce more later. In fact, to the extent we invest less now, fail to create infrastructure now, omit to develop skills or educate ourselves now, we have less capacity to produce later. That leaves a real burden on future generations.
Proponents of austerity can make ideological claims all they like about the supposed superiority of private over public spending but until we are at full employment such claims are irrelevant in any case. Without sufficient spending of some kind, there will be unemployment and lost opportunity.
Austerity brings no future gain. A currency-issuing government achieves nothing by “saving” some of its currency for later. There is no meaning in government saving what it can issue at will. No matter how much government inflicts harm through cutbacks, its capacity to spend in the future remains unchanged.
It’s very simple, really. Austerity now causes pain and suffering now for no socially beneficial purpose whatsoever.