I agree with you on Widespread, but I think Phish are tighter and musically more "out there" than the Grateful Dead. I enjoy both, having seen Phish maybe eight times and the Grateful Dead 35 or so.
I agree that Phish is definitely talented, but I want to see where they are in ten or fifteen years. If 40 and 50 year olds are still listening, that will be the real indication of their impact.