So reflecting on that, I think the core assumption that didn't pan out is that the memory / CPU ratio will grow because we'll need more memory, hence requiring CPUs to address more than 16 EiB of data (16 EiB = 16384 PiB = 16777216 TiB of data, what 64 bits can address).

But in practice, we've produced a lot more compute. The memory / CPU ratio has increased, but most growth has been from more CPUs (and generally not shared-memory ones, but ones with their own completely separate memory space).

IPv6 is 128 bit, so we do have 128 bit ways of addressing computers, but I'd say we're still a long way from a CPU needing to address that much memory as a common case. The speed of light limits how far away memory can be from the CPU for good performance, so short of some drastically new memory technology, it seems unlikely we'll need it soon for any ordinary type of computing device.