In metric, one milliliter of water occupies one cubic centimeter, weighs one gram, and requires one calorie of energy...
So let's stop right here - you think calorie is an SI unit? I have bad news for you - you have totally proved the opposite. In fact it's those
imperial units that try to be more concretely related to everyday things (sometimes successfully, sometimes not), which is exactly why you picked this non-SI calorie heresy - you felt it's
useful. This is also why they are popular.
The
real argument for SI units is not that the value of one unit is somehow more concrete, because it usually isn't - it's that when you start to calculate something a bit more complex, then you don't need to apply conversion factors all the time.