I'd say things would get pretty interesting well before that point. B=D is just the point at which a default is inevitable (unless much higher tax revenue is achievable without causing other problems).
But the problem is that the interest rates are so low now that large increases are not outlandish. 7 years ago, the rate was more than double what it is now. Looking at the graph in the article, in the 80's it was over 10%, more than 4X the current rate (which would imply 1.6 trillion in debt service).
It seems like we're making a big bet that interest rates are down permanently. That may be true, but it seems like a fragile assumption to me.