I don't believe TIW was designed to fill Signature restaurants specifically. When it first appeared, it was mostly marketed to local annual pass holders as a way to entice them to visit regularly - and not just in the parks. Unlike most resort guests, this group has other dining options and Disney wanted to capture some of that market. DVC members were added to that mix for similar reasons.
I think it will return, but not until all restaurants are open and capacity is no longer reduced.
Steve