Why does the web installer require you to unlock boot loader before downloading the OS? It seems much more reasonable to download the OS first because that takes a long time and could easily fail. Once the boot loader is unlocked, it's much harder to go back.

It's a fair question. A member of the development team would have to answer that.

That said, when you unlock the bootloader, the old OS is still installed on the device. It only gets overwritten when flashing the new OS.

Therefore, one could relock the bootloader and simply boot back into the original OS (with wiped user data).