Plone 5.x does auto resize images (at least their width and height, not their absolute density) and 8 out of 10 times it gets this right. Plone 5.1 will introduce support for "srcset" which is closer to "truly" responsive images. This makes it possible to specify specific images for specific screen sizes.
See the note in the Plone 5.1 upgrade guide about HiDPI images.
So the infrastructure will be there in 5.1. There will still need to be work done to get to sensible default implementations.