They could, in theory, have a "power saving" mode that tries to consolidate as much as it can into as few memory modules as possible (I'm currently using about 39% of memory, the rest being free and cache), virtual addresses would remain untouched and only physical addresses would change after a move and then it'd be possible to power down the memory modules that are not being used (PCIe-attached storage is slower than memory, but fast and power saving don't go together that well)