I take that there isn’t much motivation in moving to 128 because it’s big enough; it’s only 8 cycles (?) to fill a 512 (that can’t be right?).

8 cycles would be an eternity on a modern CPU, they can achieve multiple register sized loads per cycle.

If we do see a CPU with 128 bit addresses anytime soon, it’ll be something like CHERI, where the extra bits are used for flags.