Nonsense, there's no "system wide implications". Mappings are per process, and W^X is just a strategy to help harden individual processes, not the entire system. There's no herd immunity here.

JITs do not grant the ability to bypass any OS/system sandboxes. The lack of W^X doesn't do that, either. If a process opts out of W^X, such as to enable a JIT, it's voluntarily making itself less hardened, but at the end of the day this isn't any more meaningful than the program being allowed to be written in, say, C, which also voluntarily reduces the processes security hardening.