What about the implications of that belief—like eating the symbolic foods on Rosh Hashanah night? Do the Sages have formal authority to obligate that (if it counts as Jewish law) even if the reasoning isn’t correct? Is this the same as enactments whose rationale has lapsed, but whose law does not necessarily lapse?

There’s doubt already whether this is even Jewish law. But even if it is, it is void from the moment the basis no longer seems correct.