Suppose you have a really nice condition Jackson Model C, and, because you're a crazy person you've been driving that damn thing every day since you bought it last century. Unsurprisingly the Jackson is not very reliable, and so you sometimes drive it to the garage where it can be repaired. Now alas the dry batteries in your Jackson are its weak spot, and you find they must be replaced every 100 miles or so. The garage is two miles away.
After you've driven 2000 miles in your Jackson Model C, much of that time with antique car enthusiasts yelling at you that it belongs in a museum, you have replaced the battery about 20 times, each trip to do taking four miles (there and back) and so 80 miles.
I observe that you've used 80% of the life of a battery just on driving to and from the garage to replace the batteries.
Like this 55% of national debt statistic, my observation is at once true and completely meaningless.
It's a pragmatic concern inasmuch as the lenders's "faith in [the government's] ability to service the debt" in step 2 is not infinite. If you lose that faith, you have to (a) raise rates, (b) impose capital controls, (c) raise taxes and/or (d) force your central bank to monetize the debt. Each of these do bad things to the economy. The national debt isn't like household debt because the government is immortal, but that doesn't mean it's meaningless.