Regardless, it is fair to say that the average LFR player could not do the DPS challenge in 502 gear (prenerf). The other two, possibly. Now, 502 should be more doable and even 496 from timeless isle presuming perfect itemization, however players still suck.
In fact, I doubt a majority of the player base is able to do the step in the thunderforge from what I witness constantly in LFR/Randoms.
Just look at the number of players who aren't even aware of the legendary chain in the first place and who, even after sending them a link to wowhead, still don't know what's going on. (I don't know about your experience, but first week or two of the patch I got several dozen tells asking me how I got my cloak and most didn't even understand what "legendary questline" meant. I even had someone ask me if I could craft the cloak for them because it said "Made by xxx" on the item.)