Aren't the arguments for accepting PEP 463 basically the same as the arguments for accepting assignment expressions? The current syntax is painfully verbose and people use inefficient and ad hoc constructions to get around it. Better to have a language feature to support the way that people actually want to write code.
If Guido wrote that rejection of PEP 463 then I can't help thinking that he changed his perspective between then and PEP 572 and might have accepted PEP 463 if it had been proposed more recently. (I'm aware of the drama surrounding PEP 572, but still.)