There is no reason for the government to default when B=D. (If you disagree, please take a deep breath and think through the mechanics of exactly how the US federal government could be forced into default.)
In reality, B=D is likely to balance itself out again. This becomes apparent once you think through where the interest payments go.
If they are reinvested in government bonds, nothing happens. If they are reinvested in other financial assets, the general interest rate decreases, which will also pull down the interest rate on government bonds (reducing B). And if the interest payments end up with people who spend them on goods, well, that grows the economy, which increases D.
In reality, B=D is likely to balance itself out again. This becomes apparent once you think through where the interest payments go.
If they are reinvested in government bonds, nothing happens. If they are reinvested in other financial assets, the general interest rate decreases, which will also pull down the interest rate on government bonds (reducing B). And if the interest payments end up with people who spend them on goods, well, that grows the economy, which increases D.