However if the transitive-closure in the dependencies contains an sdist package anywhere in the tree, written in a way that wasn't designed to be imported on all platforms (this is rare, but does happen), then it isn't possible to resolve cross platform.

The prevalence of wheels does alleviate this to some extent. I think it is one of the early design decisions that pip can only install/resolve for the current platform and this will be very difficult to unwind. There are movements slowly in a direction that may one day make it possible to statically resolve python packages and remove dynamic dependency metadata or builds happening during dependency resolution.