> a lot more cost + a lot more (very expensive) cables + a performance hit
I think that the jury is still out on if there will be a significant performance hit. Thunderbolt is like having a direct connection to the PCIe bus, isn't it? Now, you're right about the extra cost, but I think that they will still be sufficiently upgradeable, albeit with more cost and fewer choices.
Fiercely debated by whom? I routinely set up machines with multiple 10GigE cards, each requiring 8x PCIe to function properly. 4K uncompressed RGB 10 bits video is 1,3 GB/s (2D, 24 fps; 3D and 48 FPS would demand 5 GB/s), this just doesn't go smoothly through a 10Gb/s link. I think this is quite typical of professional workstation work (vs something you could work on a laptop).
Obviously not you... I don't think that having multiple 10GigE cards is quite the norm though, at least for the market Apple is targeting. (Unless you are doing this on a Mac Pro, in which case, I don't know what I'm talking about).
I think that the jury is still out on if there will be a significant performance hit. Thunderbolt is like having a direct connection to the PCIe bus, isn't it? Now, you're right about the extra cost, but I think that they will still be sufficiently upgradeable, albeit with more cost and fewer choices.