Firewire even in professional environments is disappearing. Even firewire 800 isn't sufficient for real-time capture of 4:4:4 HD video, anyone who is serious about it has moved to pci-express based capture solutions and HD-SDI. It's disappearing off of consumer level laptops, and even Apple has started to lessen it's support for the standard.
USB 3 like it or not is going to be the next high-speed desktop interface.
What was USB 1.0 or Firewire 400 backwards compatible with?
USB came about as a standard to replace older archaic ports which were at the limit of their capabilities. DB-9 RS-232 ports couldn't really support any faster speed than 56200 baud without issues, and the protocol didn't support features like PnP which was a big deal back in 1996.
Fast forward to today, there are a number of high speed protocols and connectors available on the market today, and USB (the current standard), has not reached the limit of it's capability and is continuing to evolve to keep up increasing bandwidth requirements (USB 3).
Light Peak is a technology that isn't really new (fiber based high speed communication), and isn't really going to be needed in the next few years.
So Apple putting the port on their next generation of devices really makes no sense. If they put it on all of their laptops, it's going to end up as an useless port with no devices to attach. If they put it on one product (like the rumored tablet) as a do-it-all port, it's going to require the use of adapters or some sort of docking station to use any other devices with it.