Fiscal policy, government expenditures and budget balance

In the field of macroeconomics, the models with micro foundation have become very popular. There is one important concept in microeconomics, which is feasibility. Feasibility means that the expenditure will not exceed the income. This concept is also applied to the macroeconomic theories with micro foundation. The government often plays an important role in many of these macroeconomic theories and the concept of feasibility asks the government to balance its budget, which means the total government expenditure over time will not exceed the total government income over time. However, in the sovereignty bond market, investors do not always make their decisions in this way. The UK and US government bonds are often assumed to be risk-free equities, their 10 year yields are often used to refer the market risk-free return rate. Under such circumstance, the bond is definitely not something that investors look for higher returns from taking risk. Instead often investors tend to buy in more of such bonds to counter the exogenous risk. Then from this point of view, as long as a government is able to maintain its economic position in the world economy, investors will keep buying its bonds to counter their risk, as they assume government bonds to be safer. Often when there is an economic crisis, investors tend to buy in more safer equities, this means a government is able to gather more funding from the bond market as long as it can maintain its economic position in the world economy. However, from the feasibility point of view,  the additional incomes from the bond market have to be paid back in the future; therefore, the tax rate after a crisis will significantly increase.
Tax rate increase after a crisis is not always the case. The government can have another theory to pay back its loan. We generally expect as long as our economy is growing in long run, there will be an inflation trend. When there is an inflation in the economy, it will devalue the bonds, which means the government will be easier to pay back loans in the longer period. If a government is able to issue its bonds with infinite maturity period, then due to the inflation for an infinite time period, the value of the bonds will collapse to zero. Therefore, the government does not have to pay back its loan.
Therefore, as long as a government is able to maintain the confidence in its economy, investors will allow the government to rollover its bonds so the government can borrow loans forever without worrying too much about paying back.