We've had 800x600 as the expected minimum for _years_. Like, since it was called Red Hat Linux. It's not even a particularly egregious assumption, that's the fallback resolution for Windows 2k and later, and it's the Gnome HIG minimum design size.
That's why anaconda has a fixed-size UI, and why that fixed size happens to be 800x600.
imho, 800x600 should be used only as the last option, if resolution
detection fails, and i assume detection doesn't fail that often.