Is it? Motorola's and LG's phones are very, very thick, and the Moto G is their best seller ever. Sony's phones are slim and they don't sell. Samsung's galaxy S6 is slimmer, just like Samsung's Mobile profits.
It seems to me that Samsung should focus on their strengths and the strengths of the Android ecosystem, and being successful by conquering a nice here, a niche there... Trying to bring their flagships close to the iPhone (features and appearance) is clearly eating Samsung Mobile alive and alienating those that really enjoyed those features.
A typical Galaxy Note buyer loved the great battery life and ability to exchange batteries, the flexibility of the Micro SD and so on, no? Clearly the strategy of showing the middle finger to those users and try to conquer iPhone buyers won't work, not with Apple making such fantastic products with a very powerful and unmatched ecosystem.
I agree that there's only room for one Apple, but there's also room for Samsung phones if they focus on their strengths.