I'm not sure if his heir would be or not - Richard's pro-French policy is very unpopular, and while Richard might - just might - make it stick, his heir probably won't.
Does butterfly away the War of the Roses though. York and Lancaster may still be rivals, but the main Plantagent line is on the throne, and both have no claim to take it except "See this sword? That's my claim. You see, it's bigger than yours."