I think rise in competition from LG, HTC, Sony, Motorola, and yes, even Samsung themselves with their more diverse portfolio of devices (for example, how many people decided to skip the S4 to wait for the Note III? There were plenty here that said that's precisely what they were doing...) is the cause behind the S4's less-than-expected sales.
This is also why I think it's so important that Samsung doesn't "run away" with the Android platform. They're top dog, for sure, but they can't be the only ones in the field. I think it could be detrimental to the Android platform if we lose, say, HTC. I'm happy to see Motorola and LG making and offering serious options in the Android world. It keeps everyone competing.
I also think it's good Samsung isn't resting on their laurels. They're clearly trying to expand their brand as much and as far as they can by tying the Galaxy name to variant devices that address niche markets (the Active, for example). I get the sense they're listening -- more so than any other OEM, including Apple -- to their customers and to the media. There's clearly a response to the "plastic" argument (look at the Note III -- I think the leather is a "quick response" of sorts to that).
The S5 is rumored to focus more on better build materials. They're listening and that's a good thing.
PS. I don't foresee this affecting what Apple is doing directly. I do think, however, that Apple can no longer ignore nor pretend that larger screen devices aren't in demand.
