Tracing JITs have their downsides

I believe that indeed, it was someone from TraceMonkey who pointed out why tracing JITs don't work that well in practice: “You fall off the trace”.

But fundamentally, what people want is “faster Python”, not “a Python JIT”, and even though one's used to JITs giving massive speed boosts, the latter is not necessarily the most productive way to get to the former.