Light peak isn't a replacement for USB 3.0. To quote Intel's own page on the subject...
"Light Peak also has the ability to run multiple protocols simultaneously over a single cable, enabling the technology to connect devices such as peripherals, workstations, displays, disk drives, docking stations, and more."
Light Peak would simply be an alternate cabling, which allows for higher and less error-prone transfer rates, for existing interface technologies.
Thus one could very well say that USB 3.0, with it's high transfer speeds, and Light Peak would be an excellent match. It wouldn't seem unreasonable, to me at least, that Intel is holding back USB 3.0 to have Light Peak ready for a simultaneous deployment rather than developing the latter as a "replacement".
Just my 2c based on what little we know so far though.