Uhhh almost none of what you're saying is relevant to why there's a brick limit. The limiting factor is networking, it has little to nothing to do with graphics. The reason why we should have a higher brick limit is the same reason why we just got an fov slider: people should decide for themselves hiw they want the game to look and perform. Let's agree to disagree, and say that the same percentage of people don't use the default fov of 90 as the percentage of people who can run more than 256k bricks. (Personally, I think the number of people who don't use 90 degree field of view is around 2% and the number of people who can run 256k bricks is much, much, higher). How useful is the fov slider for the vast majiority of blockland users? Not very. But should it have been added? Absolovinglutely, 100%.
Why is it any different, excepting development time? The answer is that it isn't. The brick limit should be increased, period. Same thing with anti-aliasing.
(Also, my 7850 can run 256k bricks with full shaders. At the time of posting, you can buy it from newegg for $165, a price point that can be described as low-medium range)