"We're all each other has"

This fascinating sentence is impossible to parse strictly, because the phrase each other has become, in our linguistic consciousness, divorced from the original structure which generated it.

1) That underlying structure is something like:

Each of us has the other(s). (the plural being employed when there are more than two of us)

2) This becomes

We have, each the other(s).

3) This is generalized as

We have each other.

4) So far, so good. What happens next is that each other becomes apprehended as a fixed noun phrase, still signifying the reciprocal relationship but no longer constrained to a strict syntactical coherence. (I make no claim that what follows represents an actual historical development — just a sort of underlying logic.)

What we have is each other. or All we have is each other.
Each other is all we have. or Each other is all we have.

5) This now runs up against a semantically equivalent construction:

We are all we have.

6) But since in 4) all we have is each other, the penultimate stage is to substitute each other for we as a sort of reciprocal pronoun

We're all each other have.

7) And the final stage is to replace have, reflecting what our high-school teachers drummed into us — that each takes has.

We're all each other has.