Pet peeve here: "variable" and "mutable" aren't concepts that are useful to conflate. Imagine a function that takes a variable and prints its value to the command line. There's no mutation of the variable going on, so the variable is immutable. And yet it's still a variable because each time that you call this function, the value that gets printed is allowed to vary. This is distinct from a constant, where each time you call the function the value of a constant must be the same.
One keystroke, not two? I type l<tab> in Xcode then I'm typing in the variable name. You'll have to arrow down if it's the wrong one then hit return.
You can argue that it's really not much extra work but it all adds up. Especially when you try to code on a tablet, which I find myself doing occasionally.