It looks that WP core is improving. New functions in v4.1 added hidden H2 titles for post/posts navigations.

As for the new themes, designers should adopt good starter themes. If Underscores would have solid accessibility features and people would be encouraged to start designing using those boiler-plate themes, things would move faster.

Of course every designer can impair accessibility features while designing, but at least they would have solid base, that would still keep them on accessibility path.