Better can be enough, if it's ENOUGH better. The rule of thumb is that something has to be at least twice as good to replace a similar but already functioning system (the real costs of retraining and lost productivity during the changeover can almost never be accurately calculated, that's why there is the rule of thumb). For an academic view, see "The Fable of the Keys" http://www.utdallas.edu/~liebowit/keys1.html about qwerty vs Dvorak; and the version that was expanded into a book about Dvorak, Microsoft, and the internet http://www.amazon.com/Winners-Losers-Microsoft-Stan-Liebowit... .

ADDED: And Christensen in "Innovator's Dilemma" http://www.amazon.com/Innovators-Dilemma-Revolutionary-Busin... shows how slight improvements can gradually replace something that was originally better.