Irrelevant. They're only talking of changing the DEFAULT, not of making giving up up to 5% cpu MANDATORY.
In other words, they claim that -MOST- peoples machines are probably better of in MOST cases letting a gone-crazy SCHED_FIFO process hog only 95% of the cpu, rather than 100. Because it makes it possible to for example open a shell and kill the bugger.
The existence of special rare cases where you really want 100% is a no-argument: Simply *turn* the provided knob up to 100 for those cases then.
You're in essence saying that 99% of users should turn the knob down to 95 - so that the last 1% won't have to turn it up to 100. Which makes no sense.
Also, your example is extremely contrived. Indeed it serves as a good example of just how uncommon such situations are likely to be.