Lots of legacy apps use bit-mapped graphics instead of vectors. Xaw and Motif are both not resolution-independent toolkits but getting them to support integer scaling (with line-doubling) would be far easier than trying to implement fractional scaling plus anti-aliasing.
Apple literally doubled the resolutions of their pre-HiDPI screens for their current screens and then scaled their interface by 2x for the default OS builds, so if their interface is too big for you then it probably has been since Mac OS X came out.
Sure, and older software not designed for newer paradigms will always be a problem. All major desktop OSes currently make some compromises to handle HiDPI displays.
Apple doubled the base display from pre-Retina laptops (1440x900), but they offered a higher-resolution display (1680x1050) and that’s what I used then. I’ve always adjusted the resolution on Retina MacBooks to be the one-notch higher-PPI option.
Apple literally doubled the resolutions of their pre-HiDPI screens for their current screens and then scaled their interface by 2x for the default OS builds, so if their interface is too big for you then it probably has been since Mac OS X came out.