I've been thinking about it. Rob doesn't need to succeed Ron to be stepping up from NBC5. Sure, you could ask why he'd go from #2 at one station to #2 at another, but was we he really second banana at WMAQ? Anytime Stafford was off, Dick Johnson would cover the later shows like 4 times out of 5. Also, NBC's leadership seems high on Stefan Holt, which might set Holt up to succeed Stafford (whenever, no rush), leaving Elgas in the cold.
Back at 7, even with the new hire, I'd say Krashesky is next in line after Magers. While that might not rationalize the move, Elgas would likely front two newscasts compared to the single broadcast he left, and be on a more solid track to succeed Krashesky, in a similar time frame as if he stayed behind Stafford.