Anders Hovmöller writes:
In python code basically can't be understood at face value.
Not really a problem. The #ToddlerInChief doesn't code; the rest of us are adults and use code by consent. Obfuscate your code by seriously violating the expections for the meaning of "+" or ".denominator" and we'll withdraw consent.
Really, it's as easy as that. "We" know unacceptable obfuscation "when we see it." There's no "red line", but this is good enough.
The problem with the proposal is that it obfuscates "in a good cause" by confounding the Pythonic semantics of "= as name binding" with the natural but un-Pythonic "= as changing a variable in-place".
Somebody (Rhodri?) recently said something like "I keep coming back to the Zen: explicit is better than implicit." I think that says the whole argument against the proposal.
Footnotes:  In other languages.
 IMO, I do not speak for Guido.