The 5 Families will always be alive because any slots left vacant in the ranks of LCN will be filled by Zips and even guys from the mainland, in the case of there not being any American-bred wiseguys left to fill in. That's my guess. There's really no point in scratching the current LCN structure in favor of a brand-new Italian Mafia, it's easier to just join what's already well-established.