To be fair , most of the reason why pci isn't needed anymore is everything is integrated onto the board that once required a card. On the board they do still use the pci bus just without a slot but all the signaling , I/O , is still there.
The issue with USB3 delays was not in getting a card or chipset, the tech is nothing new, same tech is used in HDMI. It was with the cabling and connector which is extremely finicky . This is going to be a case where quality of the cable makes a huge difference the longer it gets. There are 11 wires total, 3 data pairs , 2 power, 3 ground, and each pair has to be independently shielded and then another shield over that for all the wires. USB2 only requires 4 wires and no shielding.
Expect to see some cheap cards out for it with a catch. USB3 can do error correction in hardware or software. Guess which is cheaper to make ?
Everyone is talking about the bandwidth but one area that people haven't thought about much is gamers. Many people like to increase polling on the mouse USB to increase the rate the cpu looks for changes. USB 2 works by the host repeatedly asking the device, do you have new data. USB3 doesn't use polling instead it uses the interrupt method. The host doesn't do anything but go on doing whatever it needs to do , as soon as the mouse moves data is sent that alerts the host regardless of what it is doing , the host acknowledges it and processes the data. The host is free to do other things till that data is sent.
Other major change is that usb3 is unicast. Every time you send data to devices connected to a USB2 interface , every device on that connection has to decode the data even if they cannot use it because the address of the target is mixed in with the data. So connect a mouse and usb stick on the same port and when sending a file to the stick the mouse has to decode it to see if the data is for it. USB3 sends data and address separately. So only the device that needs the data has to decode it.
USB3 also allows data to be sent out of order. USB2 has to send data, wait for a reply, before sending more data. USB3 allows data to be sent , next data to be sent, more data to be sent, before getting a reply that it was received. And the order the data is responded to does not have to match the order it was sent.
It will replace USB2 on motherboards because it still supports USB2 as part of the spec and USB2 will still be needed for devices that have really long cables and don't need the speed.