contractions - "If you don't do it, I'll". Why does that sentence feel so awkward?

Earlier today while writing a very informal email, I expressed:

If you don't do it, I will.

Upon looking at it, I realized that "I will" could be contracted to "I'll", however, this is completely unheard of and feels quite awkward. I am now quite curious, is there a reason why ending a sentence with "I'll" feels (or is) so inappropriate?

I understand that the use of contractions is generally an informal use of language so rules of usage can be a bit loose. Therefore, this is a case that to me doesn't make logical sense and I'm a bit confused. Why does it feel wrong when an "I'll" is placed at at that end of a sentence, even though such an expression is informal and not, technically, an incorrect usage of the contraction "I'll?"

Answer

Contractions can only be used in English when the thing they're contracting does not have any sentence-level stress. (Sentences have stress in them much like words do.) In this sentence, the word I carries a heavy stress:

If she doesn't do it, I will.

Because of this, I cannot contract with any of its neighbors. The same thing explains why we don't use contractions in sentences of the following sort:

Do not go in there.

I would go to the store, but I have to study.