Could it be because the RAM capacity increase over time is also logarithmic until it completely stops? This is a basic example of where you'd use a logarithmic scale. If you'd use a normal scale the line would basically be flat and then shoot straight up in 2011, then continue flat again.
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There is no denying that continuing the growth rate as it was prior to 2011 would have been nonsensical... However, it completely grinding to a halt is equally stupid, especially for the prices Apple charges. The 8GB options do not exist to be actually usable, they're just there for the marketing. Most high-end phones have long ago surpassed Apple's baseline RAM capacity, which is genuinely ridiculous (on both sides, to be entirely honest) and now that even the iPhone is catching up to it, it really is time for Apple to scratch themselves behind the ears here. Most of its competitors no longer offer 8 GB variants of their high-end devices anymore either (and hell, they usually have separate VRAM too). 8GB is just not acceptable anymore.