I had never really given the "bits need to arrive at the same time" much thought but it's an excellent point as to why parallel has limits. However, why couldn't parallel transmission be defined as serially sending whole blocks of data down each wire path? The cross talk issue is still there but if each parallel wire could transfer 1Gbps of serial data, combined that's 8Gbps and faster than the current 6Gpbs SATA spec. Perhaps we could call it PSATA?
Of course I don't relish the return of those ugly parallel cables. However, as speeds continue to increase won't we eventually have to consider something like PSATA? Won't the signal rate get so fast as to be masked by the natural capacitance of a wire? Perhaps that's when SATA could go optical but it might be just as cost effective to stuff a few more RF-isolated wires into the cable that are virtually bonded. That would immediately double the speed (or x3, x4, x5 -- depending on how many wires and how well they can be RF-isolated to prevent cross talk) without any increase in the signal rate.