Up to congress to balance budgets. Markets will force their hand if they have to (see England).
That didn't happen in England (The UK actually - England is California in the United Kingdom).
Markets are servants not master. And you cannot 'balance budgets' if you allow savings in the denomination. The accounting won't allow it. For somebody to save somebody else has to borrow. That's why you end up 'in credit' at the bank. The bank is borrowing from you. Presumably you don't want your bank to balance its budget and confiscate your 'in credit' balances.
That is precisely what happened. No market buyers for GILTS because tax cut play jeopardized future debt obligations by the government.
If for sustainable you mean a complete collapse of the economy as excess savings and excessive taxation drain the flow of money.
https://www.statista.com/statistics/200410/surplus-or-defici...