It is simple in my mind and probably many other peoples' --
The PE seat gives you maybe 25% more recline than regular economy, with other amenities that are not dramatically different. Then it should cost something like 25% more than regular economy. Not 75-100% more.
Lie flat can (to some/many) justify 3-4x price, but that is lying flat. This PE is priced wrong.
The overall opinion seems to be that Singapore Airlines should change the pricing policy on their Premium Economy product.