The original USB interfaces which apple made mainstream in the iMac? I don't know. But I do know that MacBook Pro prices with Thunderbolt are similar to without, probably because the thunderbolt controller also does DisplayPort (I wouldn't be surprised if Apple built the connector themselves, actually. Maybe ifixit knows.)

The market is different than when they put USB into the iMac now mainly because there are large amounts of cheap USB peripherals which USB 3 is backwards compatible with. I think (but haven't done any research) that the number of companies making USB peripherals now is much (one or two orders of magnitude) greater than the number of companies making PS/2 or parallel SCSI when the iMac was released. The personal computer market has grown a lot, and USB now has much more momentum than anything else had when the iMac was released. People (such as my mother) are going to buy a new computer and are going to want it to work with their existing external hard disk -- they're not going to transplant the disk to a new case. So USB, especially is USB 3 peripherals can run at slower speeds over USB 2, is going to stay for a little while. Thunderbolt is going up against an entrenched competitor, and if you compare to the HD format wars, both players worked with DVDs. If Blu-Ray didn't and people had to have two boxes instead of one, HD-DVD probably would have won. The best Thunderbolt is probably going to do for the near future is be the new FireWire.