operator* can return exactly one type. You can choose which, but metric offers many possible choices, and with floating point math on computers you will lose precision converting between them in some cases so you need to take care to get the right on for your users - which will not be the same for all users.

One return type, for any given combination of parameter types, not to mention the possibility of templating to manipulate the return type….

See, more trade offs...

honestly, I’m not seeing the problem you’re seeing