5x5 will produce mediocre results for most, but that's inherent in the philosophy, as it's a low risk, low reward sort of enterprise.
Although he has his obviously exaggerated way of saying things, it is an important concept to note. I'll always refer back to Starr in conversations to do with 5x5. Never was it intended for people to always do 5x5 continuously. And there was never the assumption that 5x5 was all that was needed to get to the advanced stage. If it was, then great! But that was an oddity (like Bernardski as noted in Defying Gravity).
The progression was always from 5x5 to a more varied rep scheme which included singles, more than 3 workouts a week, varied exercises, frequent lifting and periodisation of sorts.
5x5 is the entry level routine, a very good entry level one but still an entry level routine. Once that has produced results and people are stalling (after having considered other factors) the intention is to vary the routine. Mostly via volume and frequency as in both go up!
Why do people get stuck on 5x5? Well like anything people will form an emotional attachment to a routine and a *name* and identify themselves with it. (WESTCYDE 4 LYFE anyone?) People are also inherently lazy and would rather stick to what they know even if it isn't producing results rather than something which looks more complicated initially to understand and requires them to bust ass more.