The same combination of reasons we've been doing these kinds of heuristic implementations forever:

- people still get value off of an incomplete implementation, because we are allowed to be mindful of the issues (see, eg, UB in C++). In other words, many, many more bugs have been prevented by the borrow checker working as intended than have been caused by it working poorly. It's not particularly close.

- theory almost always moves slower than practice

The mutable state situation pointed out by the grandparent poster was a famous issue in typing SML under mutability that required serious changes in the language, see Section 4.2 here: https://smlfamily.github.io/history/SML-history.pdf

These were Turing-award caliber people figuring it out and, according to your definition, "yolo"ed it. This stuff is _hard_, and these driveby comments truly aren't helpful.