Normally, my starter only remains my strongest Pokémon on the team during the early parts of the game. After my team starts to build up for the Elite Four, I have a habit of over training them all and keeping them all equally at the same level. So, only after a certain point will my starter become equal with the rest of the team.
Though, after reaching the Elite, the levels start to become unbalanced and I leave it all to chance for who ever becomes the strongest on the team. Most cases it isn't my starter although I don't doubt it may be possible for Quilava to be the highest level this time around.