> they are genuinely worse in terms of construction
A 406 costs ×20 more (£1600 vs £80).
Does the 406 have cleaner lines? Yes.
Is it comfier? The 406 doesn't have a cushioned seat so, maybe or maybe not.
Will it last longer? Probably. But the IKEA one comes with a 10 year warranty and unless you treat it badly it'll probably last far longer than that.
Hard to argue that the IKEA POÄNG isn't great value for the people who have other priorities.