I tend to think that Samsung's phone sales have given them an inflated impression of themselves over the last couple of years and a tendency to rest on their laurels while piling endless bloat onto their devices and seeming to spend almost nothing on designers (or they just don't hire the right people) Samsung have simply remained the same, while everybody else has caught up or passed them.
People only really bought Galaxy phones because they were a viable cheaper alternative to the iPhone early on, they were the only real Phablet game in town and because Samsung spend more than anybody else on marketing. Now there are better AND cheaper alternatives, like the Nexus and the cheaper chinese Apple clones like Xiaomi which actually do realise that the look and feel of devices is actually far more important than the specs to the vast majority of people. Instead of admitting that they got something wrong, Samsung instead say that the market is weak. This is the same market that is buying iPhone 6 Pluses faster than they can be made. The problem is not the market. Samsung's audience for phones either don't have a lot of money or don't want to spend it on Samsung's devices.
Nobody wants to feel ripped off or mocked and the Galaxy S5 does not look, feel or perform like a flagship product and especially not a premium answer to any iPhone. Out of the box, without any tweaking, the phone is slow and the camera is, I would almost say broken, by its implementation of IS. This Alpha is even less impressive, simply because this is Samsung very vocally going 'all-out' to quell it's critics and ending up with a plastic-backed lemon. Despite being available for many weeks I have yet to see anybody walking around with one of these.
Samsung mobile will decline until they drop their prices or innovate something which people find genuinely useful or that just plain work reliably. Everybody with > a galaxy S4 uses 'Air-Scrolling' every day don't they?