Wouldn't it be nice if we could measure this and find if there's a point where colder water doesn't provide a benefit, or rapidly diminishing returns?

Wouldn't it be nice if there was evidence that 14C water conferred the same benefits as 4C water?

I know it would be way easier for me to suggest this as a habit to family if I didn't tell them, "yeah you just use water as cold as it goes, even in winter when your pipes are about to freeze".