That might be, but with the right Apache, nginx, varnish (take your pick) module, this friction will be reduced to saying something like:

    lower_res {
      Request /lowres/*.jpg
      Provide /highres/$1.jpg
      Downscale 50%,50%
      Cached_on /mnt/cheapdisk_that_can_go_away_at_any_time
    }

I'm not aware of such an existing module, but I'm sure a good one will appear within the next couple of years of it does not yet exist.

Compared to developing and deploying a new progressive format (across users, web servers and web browsers), the effort -- both in developing this module, and in configuration, is negligible.