Depending on who you ask,the Western Empire did fall eventually,just not in the fifth century.The Visigoths took over the entirety of Gaul,Britannia and Hispania,and subsequently proclaimed themselves the real Roman Empire to appease their Roman subjects.It was a miracle that the Romans in the west managed to form a defensive perimeter and hold onto Africa,Italy,Pannonia,Noricum and Dalmatia until the eighth century.Eventually,they were conquered by the Visigothic Roman Empire,but I guess what would really happen if the true western empire didn't survive until then would be the complete collapse of civilization in the west.The Visigoths became so thoroughly Romanized by modelling upon the western empire that there was no difference between them and the Romans either in Italy or in the Eastern Roman Empire.Eventually,most of the Western Roman population just saw the Visigothic Roman Empire as just another division of the Roman Empire not much different than during the days of the Tetrarchy,such that the Romans of the Western Empire had no difficulty adjusting to Visigothic Roman rule when the Western Empire was eventually conquered,and saw the Visigothic Roman Empire as just the continuation of the Western Empire.