? Too big a change? Too much magic? Maybe more difficult to read or parse. The argument "It's not clear that the function is a template" isn't there any more, though, so it seems reasonable to me.
You actually hit on the exact reason; your change is significantly harder to parse. C++ allows you to alias typenames and variable names, so you wouldn't know if `int foo(T);` was a declaration of a function that take a type `T` or a function that has an implicitly typed parameter called `T` until much later in the parse. Adding auto makes it clear this is a declaration of a variable. That kind of ambiguity already exists in C++[1], but is widely regarded as one of the things that makes parsing C++ extremely hairy.
Interestingly, I remember hearing about this exact problem in the context of Swift's closures; if you want type inference, you need to leave off the parens around the parameters. This is mainly done (IIRC) because it's easier for the parser.
