This is an argument for having less memory on a hardware level. But once the DRAM is there, it uses power, whether or not it stores useful data or useless data.

There's a reason why we say unused RAM is wasted RAM.

Powering down unused physical RAM is absolutely a thing on some systems. For one thing, it's required if you ever want to support physical memory hotplug. The real issue however is that the gain from not doing DRAM refresh is clearly negligible: it's no more than the difference between putting a computer to sleep (ACPI S3), or putting a phone to sleep in airplane mode - and powering it off.