I think the real criteria is roughly “how much benefit do we get, for how much cost”.
Cost can come from many places. It can come from complexity of the proposal, it can come from backwards compatibility concerns, it can come from the long term overhead of maintaining and explaining a feature, or many other avenues.
My read of that post is that, the opinion of the SC, is that although the cost is small in implementation complexity, it’s much higher in long term overhead of adding yet another way for dependencies to get located and environments to get managed. They weighed it against the benefits, and found the cost too high compared to the relevant benefit, as the PEP currently stands.
This is generally a reasonably sound structure for making decisions in Software Engineering (and in general tbh). There’s very little in life that comes with all benefit and no cost, so every choice should be weighed with that in mind.