There are definitely things to be said about cost-benefit analysis acting as a Great Filter for interesting art, but the real reason "kill your darlings", as advice, should fucking go is:
quick, what does it actually mean
no, like, specifically
what actionable thing is it telling you to do
It's a neat-sounding phrase, isn't it? Clever. I bet someone was really pleased with themself for it, and as advice it's bad because it sounds like it's telling you to do something, but the second you ask what that is you're on your own. It's beautiful and actively bad at its job
making it, ironically, a darling that should have been killed