First of all -- it was DiMaggio, not Williams, who holds the record for the longest hitting streak.
Also, it should be noted that while nobody has hit .400 since Williams, a few have come close enough that it is not improbable for someone to do it again. Rod Carew hit .388 back in 1977 or 1978, George Brett hit .390 in 1980, and Tony Gwynn was hitting .394 when the baseball players went on strike in 1994.
This last item, by the way, is why I will never attend a major league baseball game for as long as I live. Denying fans the chance to see Gwynn make a run at .400 was an absolute crime.