Removing the GIL has nothing to do with making breaking changes in the language. It's also an enormously more complicated task, one which hundreds (thousands?)

Removing the GIL has nothing to do with making breaking changes in the language. It's also an enormously more complicated task, one which hundreds (thousands?) of developers have spent time on over a decade (two?). I think you might be thinking that the impediment to removing the GIL is user code. It is not. The impediment is that CPython - the standard Python runtime - was designed around using the GIL, and it's extraordinarily hard to remove it and still meet the existing single-thread performance goals. There are also ancillary issues surrounding user code - particularly modules written in C, I believe - but the runtime itself is the biggest issue.