> Go needed to allow multiple outputs because functions can accept multiple inputs.

I don't think that's the right conclusion, unless you have any source or insight?

It would explain why you can directly cast the multi-return-values into parameters when calling, but... that doesn't seem to fit go at all.

What I would think is that "go needed multi-return to be able to return errors along values", and the mentioned feature is just some vestige. It's not like go ever implements anything for the sake of consistency or correctness.

> I don't think that's the right conclusion

It is the "right" conclusion in general. Multiple input arguments without multiple output arguments is frustratingly awkward. I can understand the appeal of your suggestion of only accepting one input and returning one output, where the passed value might be a tuple, but I expect there is good, pragmatic reason why virtually no languages, even those with a proper tuple type, haven't gone down that road. Once a language accepts multiple arguments, though, it needs them in and out. Otherwise you can't even write an identity function. If you can't write an identity function, is it even a function?

> "go needed multi-return to be able to return errors along values"

If there was some specific reason for its addition, beyond the simple fact that it would be crazy not to, it was no doubt for map presence checks. Without that, it is impossible to know if a map contains a given key. Errors can be dealt with in other ways. It wouldn't strictly be needed for that purpose, although it turns out that it also works for that purpose, along with a whole lot of other purposes. Not to mention that we know that the error type was a late-stage addition, so there is strong evidence that errors weren't a priority consideration in the language design.