Interface Speed versus I/O Speed
So here'e the deal... the interface speed doesn't really matter unless you have enough I/O to push it. Most users only have a single hard drive connected to their Airport Extreme. So regardless of the Interface (USB 2, USB 3, Firewire, or even Thunderbolt) you are limited to the drives performance and I/O. so it won't really go any faster.
Also, if your using the Airport Extreme in a wireless manner, then your WiFi network is a slow component as well.
All this discussion about theoritical thruput doesn't matter unless you have 6-8 SSD's in a RAID 0 stripe, then you might actually hit the ceiling of the interfaces. Ever actually copied 800MBs on your firewire connected drives? I dont think so. Ever copied 480MBs on your USB 2 drive? Didn't think so either. And you will certainly not claim to have copied 5GBs on your USB 3 drive. If so, you're just plain silly.
The only way to get those numbers is the RAID multiple drives together to hit those numbers. So unless you have (20) 6Gbs SSDs, moving an average of 500Mbs (500 x 20 = 10GB) then you probably wont hit the ceiling of a thunderbolt connection anytime soon either.
Summary:
Even if Apple added these new ports to the airport extreme, you would only see small improvements on the speed. That's a fact.