From OP: "makes a solid argument for why an iPad retina display must be pixel-doubled -- i.e. 2048×1536 -- and not some intermediate resolution (just as was the case for the iPhone 4 before it). Anything else means every single existing app either has to re-scale art assets -- resulting in a fuzzy display -- or let them appear at a different size on-screen -- resulting in usability problems as the tap targets are resized. This is because every single existing iPad app is hard-coded to run full screen in 1024×768."
Doesn't this suggest that Apple is getting bitten by backward compatibility to the mass of pre-existing apps, just like Microsoft got stuck with the mass of existing software running on Windows (and also actually users who get too used to existing UIs/UX)?
Not exactly. This theoretically only pertains to bitmap UI elements (mostly icons). Sadly bitmaps are everywhere, notably because they're so cheap to create and render compared to vector, and more computation implies more energy. I suppose one could generate a cache of rasterized vector UI elements to cut on subsequent rendering.
Apple tried to bring resolution independence to Mac OS X since quite some time, and in all honesty it worked well... for vector stuff. It broke in varying ways across iterations of it every time there was a bitmap involved, in which case they were at best either blurry or unscaled. There's no miracle, unless you generate bitmaps for numerous multiple sizes (like in icns files, where they range from 16x16 to 512x512, downsampled if scaling is needed, like on the Dock), initially small bitmaps will just look bad unless you use a 2x factor, in which case you will at best have no improvement (but no loss either) over a non 2x screen, or you have an uncanny effect when a 'fat pixel' bitmap stands near a 'thin pixel' vector curve. Anyway as noted by robomartin, things are sufficiently bloated already not to include full-scale 16->512 bitmaps.
What's more 2x is computationally way simpler and much less costly for everyone. The only non-hackish, seriously viable alternative is to go all the way vectorized. A typical case of 'less is more'/'worse is better' if you ask me.
Doesn't this suggest that Apple is getting bitten by backward compatibility to the mass of pre-existing apps, just like Microsoft got stuck with the mass of existing software running on Windows (and also actually users who get too used to existing UIs/UX)?