The central thing about all these conventions is they developed in the absence of good union mounts. In Plan 9 there is no $PATH: all such things are bound into /bin. This works both because Plan 9 has very good union mounts, but also because those union mounts can be localized to a specific process and as a result are user accessible. I seem to recall some work was done to make this possible on Linux but It didn't get accepted into the main kernel because it is at odds with how we see how a Unix system should behave.

It would be interesting to see a Linux distro that embraced this.