I agree that srcset has its advantages, and like I said in the post, I expect that it will make a fine solution in the long-run, and I'm glad this problem is being addressed.

One minor point, however, is that the strawman <picture> proposal was meant to be equally backward compatible through the use of a child <img> tag that would only display in unsupported browsers, as <video> works today:

<picture alt="cool-image">

  <source src=hires.png media="max-width:1920px">
  <source src=midres.png media="min-width:800px">
  <source src=lores.png>

  <!-- fallback for browsers without support -->
  <img src=midres.png alt="cool-image">

</picture>