Preferment yeast vs bacteria

My understanding of preferments is that the preferment allows bacteria to build up acid and increase flavor. The yeast itself does not contribute much to any additional flavor. If this is correct, why not just do a very long autolyse instead? The bacteria will still develop and will not have to compete with yeast for food. You also won't run into the same risk of over fermenting and having the yeast consume too much of the starch.