Arbitrary scaling makes images blurry, even vector graphics. It's a difficult problem to solve because computer graphics are highly aliased - they have many details only one pixel wide.
We have solved this problem with fonts, which have hinting and subpixel antialiasing. But we don't have software to do this for graphics. There is no image format with hinting, and no rendering library to do subpixel antialiasing. Hinting is difficult to do well, fonts are hinted by hand. It is a very time-consuming process and most graphic artists will not be good at it.
If your screen is sufficiently high-resolution, say 300+ dpi, you can get away without these tricks. But if you have a 140 dpi screen, and you want to scale everything 150%, it's necessary or everything will look blurry.
If you only allow pixel doubling, none of this is a problem. You can scale any graphic by 2x without ruining fine detail like thin lines.
How hard would it be to just ignore the physical properties of the display?