If JIT is a good thing for Python, why don't just compile to Java or .NET bytecode and use their already optimized infrastructure?
It beats Python on performance, supposedly, but compatibility has never been great.
This JIT approach is improving the performance of bits of the interpreter while maintaining 100% compatibility with the rest of the C code base, its object model, and all the extensions.
> The big downside with a “full” JIT is that the process of compiling once into IL and then again into machine code is slow. Not only is it slow, but it is memory intensive.
Maybe Microsoft don't know yet how to sell this thing, or maybe they are just boiling the frog. Time will tell. But I'm pretty sure your question will be repeated as soon as people will get used to the idea of Python on JIT.
Just a guess at the pitch.
Microsoft developed both JScript and Node.js. They could've continued with JScript, but obviously decided against it because JScript didn't earn the reputation they might have hoped for. Even if they invested efforts into rectifying the flaws of JScript, it would've been just too hard to undo the reputation damage.
Microsoft made multiple attempts to "befriend" Python. IronPython was one of the failures. They also tried to provide editing tools (eg. intellisense in MSVS), but kind of given up on that too (but succeeded to a large degree with VSCode).
The whole long-term Microsoft's strategy is to capture and put the developers on a leash. They won't rest until there's a popular language they don't control.