grammaticality - Is there an exception to the prohibition against ending a sentence with “ ’s ” at work here?

The ’s can be used as a contraction representing a weak, unstressed word that is not pronounced. It allegedly cannot occur in sentence final position.

• She is not ready, but he is.



• She’s not ready, but he is.


• She’s not ready, but *he’s.

The last one is not grammatical there.

Similarly, here the last one is not grammatical:

• He has not started yet, but she has.


• He’s not started yet, but she has.


• He’s not started yet, but *she’s.

This answer claims this is because you cannot end a sentence with a weak form. It backs up its point with reasonably scholarly documentation.

However, this does not seem to be invariably true.

• Shall we go to the movies? Yes, let us go.


• Shall we go to the movies? Yes, let’s go.


• Shall we go to the movies? Yes, let’s.

Unlike the third example in the first two sets, here in this case the third example is indeed grammatical, despite ending with a weak form.

Is this an exception to the rule, or is there another rule at work here?