In general, providing sensible defaults is a good thing.
The overwhelming majority of machines should be configured so that a bug in a single SCHED_FIFO priority task cannot completely lock the machine up hard.
That makes it a sensible default in my book.
Okay okay, so you can argue the KERNEL should default to 100%, whereupon 99% of the distributions out there should default to turning the knob to 95% or 90% or whatever.
But -someone-, earlier in the chain than the end-user, should change their default. It's not reasonable (as today!) to expect Joe User to know that he needs to turn the knob to avoid hard lockups on stumbling on a pulseaudio-bug or whatever.
I still think the extremely few projects that DO need true 100% can turn the knob themselves; are you aware of even a single real such project ? (as opposed to theoretical constructs like the one in the grandparent)