It's not the new features of Python that make it hard to optimize; it's the fundamental dynamic nature of the language that was there from day one. Syntactic sugar doesn't have an impact one way or the other on optimizing Python.

The new features aren't just syntactic, they're also new libraries that come standard with CPython, etc. If you want to implement a Python JIT that people will use, you have to match everything CPython supports. Furthermore, since the people behind CPython don't care about JIT, you also can't count on them not adding language features that will break optimizations present in your JIT. You can't count on these being just "syntactic sugar". Even if you could though, in order to keep up it means you have to use CPython's front-end, or constantly implement every syntactic tweak CPython does.

Lastly, AFAIK, CPython's FFI API is actually more of a problem than the dynamic semantics of the language. You can expose Python objects directly to C code. That makes it very hard for a JIT to represent said Python objects in an efficient way internally.