Then fall back to downloading the full OS image. Then the worst case would be about the same as today, while the common (99.9%+) case would be fast and bandwid

Then fall back to downloading the full OS image. Then the worst case would be about the same as today, while the common (99.9%+) case would be fast and bandwidth-cheap. (Optimization: hash tree, figure out which specific files are broken and download only those.)