Why meatgrinders are terrible for creating backstory

[KorvinStarmast]

lol, how is that a meatgrinder, by any stretch of imagination?

It's the same pattern. You go in: if you die, you are done.
You come out, and you go on an adventure (and through the trauma come out stronger).
The distinction, which I mentioned, is that Raistlin did this alone not as a part of a group.
See Dragon magazine 83, The Test of the Twins.

Raistlin heard Par-Salian sigh wearily.
"I should be the one to apologize, old friend," he said. "Forgive me. There is trouble coming upon us that the world may well not survive. This choice has been a heavy burden upon me. As you know, the Test may well prove fatal to the young man."
"It has killed others more worthy," the master murmured. Raistlin crept away.
-----------
"Your brother cannot stay," the Mage admonished softly.
"I am aware of that, Great One," Raistlin replied with a hint of impatience.
"He will be well cared for in your absence," Par-Salian continued. "And, of course, he will be allowed to carry home your valuables should the test prove beyond your skill."

What happened during that Test - the traumatic thing - had ripple effects on the rest of the stories/adventures.

As a comment on NPC hirelings, those have always been a natural pool of replacement characters. The idea that you wouldn't care about them is a result of multiple passes of de-emphasizing them as a player resource and as people.

Yes. As those incremental changes were adopted, it relieved players of having to think about someone other than themselves. (And it also removed that pool of ready to go and narratively valid replacements).