Counter-argument to changing the return type of a function with var: You can change the return type of the function to another type that might break some assumptions later in the code.

For example, if the code assumes it has a type Foo with a length field, and you change the return type of tha function to a Bar without that length field, the compiler will complain that Bar doesn't have a length field, rather than complaining about trying to assign a Bar to a variable of type Foo.

Even worse is if the Bar type changes the symantics of that length field (perhaps going from the number of elements to the max index of elements, causing an off-by-one error), the code could break silently.

That's mostly a strawman argument, however it is worth noting.