Johnson has definitely declined from his peak, but his peak was something like 9% in the polling averages, and the polling averages now have him at about 6%. So sure, that's a decline, but not the kind of "crash" that some were predicting six months ago when some were saying "Oh, his ceiling is going to be something like 2%."
Most polls still have him north of 5, yet when we get polls like this that have him below 5, everyone starts jumping on it like "See, I told you Johnson was going to crash and burn!" In the threads on polls that still have him in high single digits, there isn't a corresponding chorus of "I guess Johnson isn't going to drop like people were saying."
Third parties typically decline even more on election day itself than in the polling leading up to election day. Nader polled at 2.3% in 2008 and he got 0.6%. Barr polled at 1.5% and got 0.4%.
No doubt Johnson and Stein will improve on their 2012 performances, but at this point I'd be shocked if the former cracked 3% and the latter 1%.
