I don't think your solution would work. People could just step back and forth within range to get a few hits and thus get XP. Not to mention that having to take damage from a mob sorta ruins the whole point of root/nuke/calm tactics. Just cause someone's able to take down a mob without getting hit doesn't mean it didn't take a lot of effort and risk, and that should be rewarded.
The main issue for FC wasn't even the stationary mobs. It was being able to hit mobs beyond their field of perception and them thus being unable to strike back. I've yet to see this happen even once, but I'm willing to take FC's word on the fact the problem exists. The easiest solution would still be to simply break target lock if it's unable to target you back.
Another solution (if any is needed) has to be found for stationary mobs. But since that isn't a big issue, and it was there from day one without anybody making a deal out of it, I'd say just ignore it for now and reinstate range with the above mentioned fix.