Out of curiosity, when did the Roman culture die out (by that I mean completely evolve into the various Romance Cultures)?
A descendant, a Count Syagrius, of the Rex Romanorum Syagrius, a ruler of Roman holdout in the north of Gaul, was an envoy to the Byzantine in the late sixth century. A further descendant was a donor of a Benedictine monastery in northern Italy the late eight century. A genuine ability to trace a verifiable Roman ancestry as opposed to a legendary Roman ancestry which some medieval lords might claim, might betoken some cultural survivals. However, once Anicius Faustus Albinus Basilius held the last consulship for the city of Rome, once the Senate disappeared as something recognisable to Romans, once the civilian life of a Roman aristocrat became impossible, when Roman nobles became indistinguishable from Germanic military aristocrats, a Roman culture could hardly survive. It's hard to see much surviving the mid seventh century, certainly in the west.