I'm not saying this to start a fight, so... don't freak out.
I'm somewhat curious where you found this definition of "Bit Rot", IE, how this nickname applies to physical failure of a DRAM. The standard definition of "Bit Rot" refers to corruption of *data* stored in "static" memory technologies, most often EPROM/Flash chips, but also SRAMs storing data for long periods without refresh. In all these cases it describes the data loss that results when the stored charge denoting a bit "leaks" enough to change a "1" to a "0". The chip itself may not actually be damaged but the data has become corrupted and the only way to fix it is to write a new copy of it back to it. (A nontrivial process in the case of an EPROM.)
(Reference: A description of Bit Rot in EPROMS)
The term is also applied to data corruption on disk, tape, and other magnetic storage devices due either to mutual interactions of the magnetized bits causing it "erase itself" over time, or because the media itself literally degrades, or "rots". (The physical degradation definition also applies to, for instance, data stored on aging CD and DVD media.)
"Bit Rot" is a great term to apply to DRAM chips failing over time, but again, I'm curious if there's actually some electrical/chemical/technical phenomenon you're describing with it. (A link would be great.) Most of the references I can find on aging effects on semiconductor memories suggest that said devices have a very flat degradation curve once they're past an initial "breaking-in" period. An implication derived from that is that it's possible the memories that "fail" in Macintosh 128ks haven't actually "failed", but were simply marginal in terms of output response to begin with. If that were the case then perhaps there is something else in the support circuitry on the Mac motherboard that *is* more prone to aging that is now failing to work with said marginal devices. (Top candidates would probably be filter capacitors or parts of the power supply. Capacitors in particular are something that seem to suffer from periods of idleness.) It would be an interesting experiment to try the "bad" chips in something else that takes 64k DRAMs, say an old PC expansion memory card, and see if they fail RAM tests there as well. (It's also possible that aging in other components is subjecting the mediocre RAM chips to out-of-spec voltage surges and "killing" them, in which case it may only a matter of time before the more robust replacements or other chips on the board start to fail.)