Actually it was rather possible given that the colonies were going about the war in a herky-jerky fashion where Metacom's goal was very simple indeed. The joint power of the New England colonies won by an extremely narrow margin, thus a Native victory would also be by a relatively narrow margin. What this would arguably do is force an evolution in British colonial policy as now Spain would not be the only state to have its colonization ventures in North America to go horribly wrong.