Errr, they've sold tens of millions of these phones - I'd hardly call that a "niche market"
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And here we go again, minimising the success of Samsung. You can't help yourself.
What defines a niche market? When a form factor sells a very small fraction of the overall smartphone sales, is it not a niche product? How many smartphones have sold over the last 12 months, and how many of them have been above 5.5 inches? Is this category not a niche category?