It is pointless to think in terms of the SATA port needing to be faster than the real, theoretical potential of any future device connected to it, because it never can be, parallel (in time, not computer busses) technology developments will always make it true. Short bus low latency DRAM caching for example, will always be faster than an extended length serial bus, they can't just "choose" to make SATA faster because it is impossible and always will be unless they went back to paralleled (serial) busses which is the opposite of what everyone who fawned over SATA's early benefits wanted.
600MB/s is enough. Even SATA2 is enough. Something has to be the bottleneck, it would be silly to make everything constantly change and that at higher expense for everyone just to try and gain a tiny percentage in an unusual situation. That's why more exotic connect tech exists, SATA3 is supposed to be the cheap solution for the masses.