Could be a big deal; if Thebes stays dominant there is a stronger state around which to rally. However, the Spartans, if they are beaten, are largely a spent force politically, so they may not help much.
In addition to Macedon never conquering Greece, it could also have other interesting implications. For instance, assuming he survives long enough Phillip may have sought Epaminondas as a tutor for Alexander. It's unlikely, but could have far-reaching implications