What can tb do that express card could not?
The best part about EC was that there was no "handshakes" or any other software layer nonsense like you get with Thunderbolt, it was just plain old PCIe and it just worked. Every device you plugged in was fully "native" to the system and didn't require any screwy stuff to work right. SAS controllers just worked. 4x network cards just worked. Serial cards just worked.
These days, my old Dell workstation is on its last legs, it's looking like EC is a dead end. It's a shame, because I doubt we'll ever see that level of simplicity for connecting random devices to our computers again. Yes, the connector sucked and the form factor was awkward, but it really had a special place to me.
[1] https://images-fe.ssl-images-amazon.com/images/I/51cyhJ95LsL...
Its real replacement is Thunderbolt though! That's basically PCIe.