>An afternoon to write, and then a year or more to work out all the bugs & corner-cases, write the documentation, and convince everyone to switch.
For a JSON parser? I think you overestimate the difficulty in implementing one. If anything, a parser for such a format is one of the easiest things to know you got correct -- there are extensive test suites.
As for switching to it, Python users already use several JSON libs anyway, so it's not like adopting another would be any great deal.
For the other libs, like numpy etc, yes, it would be more effort, but nothing unsurmountable. And speed is a far greater hook to get users to adopt that version of Python than "we now got unicode all over and fixed a couple more annoyances".
Writing the code is rarely the hard part. There's a whole ecosystem around libraries and languages that takes time to build.