Why Sidebars Suck
When you use a shifting layout, you’re betting that a given page element works equally well in two different positions.
If related content works below the main content on a small screen, why not on a large screen? Are you worried about the fold? If so, why aren’t you worried about it on small screens? (Also, users scroll, so don’t worry about “the fold” on any size screen.)
If you’re just trying to fill white space on a wide screen, don’t. White space enhances the readability of your site.
And another thing, resize your window (if you’re on a wide screen) and notice how awkward the content gets right before the breakpoint. I know, I know, that’s what breakpoints are for, but why have awkward layouts at any size when you can have one, graceful layout?