Think of it this way: trying to make Indyn<T> work would lead to defining the semantics of custom DSTs, which would then lead to defining the syntax. That doesn't necessarily mean stabilizing it sooner, but it does make it easier to reason about with an actual implementation.
As for the Box digression, we consider those inseparable. We've already argued about it.
Sometimes you just need to let the implementation shape the syntax/features you wanna create. Box is special in that you can move in and out of it, despite it being a Drop type. So one should use Box to shape ?Uninit types and the stuff around it. Make an Indyn<T> and let it shape custom (thin) DSTs.