This is a complex matter. Vector graphics scales endlessly, since it's by definition independent of physical pixel matrix. As long as it is accompanied by a proper antialiasing, angular and curved edges look smooth. Raster graphics, on the other hand, depends strongly on physical pixel size. To make a simple example, the icon:

(viewed as gif, not ASCII) is pixelart. Hence, if you have too high DPI, you won't see individual pixels, making it hard to resolve.
In the golden era, we had
pixelart, which I both made and use to this day. This stuff uses indivudal pixels e.g. for creating fonts with a rule, that there can be no antialiasing (blurred pixels) and no line can be thicker than 1 pixel. Sadly, high-DPI screens are/will render this nonexistent.
Too bad. Because for example RBS, with his 19'' 1280x1024 has no problem enjoying the beauty of this here:

while pianowizard can forget it. Imagine the toil of this graphics' creator, now it's doomed.
This is a multilayered tale of mathematics, graphic artists' life, programmers' work and redistribution of work in graphic arts. Some apps have pixelart+static raster images (menu icons)+vector (svg icons). Only part of this scales, and there's nothing more than maths to it. As we advance, DPI rises, some of this content scales well, some not at all.
I'm amused by claims that "third party" content doesn't scale. Pff... if someone took his grandpa's photo in 1999 with then-state-of-art 1MPiX camera, there's no way this will ever scale. It will be a million pixels, whatever DPI you view it on. Watchable on older stuff, on retina it will turn into a postage stamp. Or, if 'scaled up' (which would be pixel enlarging) turn into pixel-salad. In other words, 99% content of the internet is "third party" as in, it doesn't belong to the corpo-troika (google,ms,apple). Good luck scaling that. So people basically speak about scaling text, and this alone. As always with 'progress', we probably gain something. We loose a lot, too.