It's not that creating the product page would be any more complicated than it is now. What I meant is that the product page would have to have extra functionality and thus would be more complex to view and navigate for the user.
i.e. we would need a call to action for "tell me when restocked" AND "tell me when low in stock". My view is that there is no way a user can tell when something is restocked until after the fact, so letting them sign up to those email alerts is important. But, a user who wants something, but wants to know that the stock levels are rather low, can (if they want it so much and can be bothered) monitor the product page.
