First off, I see nothing in MacRumors' iPhone 7 rumor rundown to suggest that the 7Plus is getting an upgraded display. The 6 has a 1334-by-750-pixel resolution/326 ppi display, the 6Plus is 1920-by-1080/401 ppi. No hints that will be changing, no hint that the 6Plus has been hampered by the lack of RAM.
You're also confusing video playback standards with screen resolution. 1440p isn't even a viable standard - show me any significant content distributed in that standard. The only step-up that would please anyone is a move from 1080p to 4K - 4096x2160. No hint of that happening. Besides, video playback isn't heavily dependent on CPU RAM - you should be screaming for more video RAM.
3GB is hardly unmentioned in the iPhone 7 rumors. Again, look at what MacRumors has to say about it. However, the reason attributed to a possible move to 3GB RAM is the demands that may be placed upon the system by a two-lens camera system. Even that speculation is of dubious value, as, again, the better place to do that sort of work is the GPU, not CPU.
This is just another "I want more RAM" thread. More is better. More is always better. Even when there's no demonstrated need for it.
And how many times does someone have to say "cheap-out" in regard to Apple, as if, somehow, the cost of components does not trickle down to the selling price of the product? You can wish that Apple intentionally takes a cut in its profit margin, but why should any business do that, unless there's good reason to believe they will more than make up for that cut in increased sales? Just how many additional iPhone 7Pluses do you think 3GB of RAM will sell?
A company has to deliver their product at a price that appeals to the public. A fair part of the art of designing electronics gear is to spend money on components that will sell more product. Which sales pitch is going to sell more iPhones, "Look at what this new, dual-lens camera can do," or "We upped the RAM by 1GB?" (And I don't mean, "Which sales pitch will the person who hopes for 3GB prefer," I mean, "Which sales pitch will tens or hundreds of millions of non-techie consumers respond to?")