Which probably is the reason why the premium economy has become a big market success, it is simply cheaper than economy class...
The market has demonstrated that if given the choice, people are willing to pay a premium for a better product. If however the product offered is inconsistent and you never know what you really get for your money, then of course everybody opts for lowest cost.
Those airlines who explicitly offer the A380 as premium product have succeded to sell the more expensive tickets.
What percentage of the seats are "premium" economy vs. regular economy? 20%? How many of those "premium" economy seats are actually paid for (as vs. frequent flyer upgrades?) Personally, if the flight is more than ~2 hours, I'd spring for premium economy - except that I don't need to. Due to my FF status I nearly always get upgraded - which also tells me not that many people are actually paying for it or there wouldn't be room to upgrade me. The market has demonstrated that once they reached the bottom, a few people would pay extra to get off the bottom - but the majority don't.
If the airlines could routinely make more money flying the A380 than big twins, they'd be buying more of them. I can count the number of airlines that have done that on one finger.