...(unless the website adjusts the amount of content per line in a webpage based on the resolution of the device it is viewed on, then all bets are off on how the website will react).
I agree with your post, but just to address this last point, I think Safari always report the screen width as 1024 pixels on every iOS device, so websites shouldn't treat the 4.7" differently.
That means that the content will be the same on a 4.7" or 5.5", but proportionally larger which is I think what we all want (full webpages are barely legible on a 4" iPhone without zooming).
I think that a lot of those who complain about the lack of PPI on the iPhone, saying "I can see the pixels!" are the type of people that try to read full webpages without zooming. Since the text is very small when viewing a page like this, it is easier to notice the pixels. For very small text like this, a higher PPI would make a bigger difference.
But on both the 4.7" and 5.5" iPhones, webpages will appear bigger, including the text, which will get much more defined and easier to read even at the same PPI, pixels will be much less noticeable.