"As I've understood the concept for something like a decade, PIPs are almost universally understood to be a soft form of firing."
FWIW, they've never been at any company i've managed at, or org i've belonged to :)
I certainly believe such companies exist, i'm not stupid.
I just am not sure I believe they are as prevalent as you do.
The only question in the room (from either HR or the manager) has always been "how do we help this person get better".
In fact, there were cases a PIP was decided against because it wasn't going to be effective in helping.
We simply offered fork in the road instead.
As for "sabotaging relationships", in every successful PIP i've seen, the person is still working at the company years later.
So ....
Apparently i'm just very lucky ;)
(which is, of course, within the realm of possibility)