The main problem was the loss of technological and economic progress. Medieval Europe was basically a set of regions with their own mafia don. However, the Dark Ages is a misnomer. Things were still changing albeit slowly.
In southern Gaul, Spain, and Italy, Roman landowners survived by coming to terms with the migrants. But to suppose that this was a voluntary process - as some of the revisionary work done since the 1960s has supposed - is to miss the point that these landowners faced the starkest of choices.
As the central Roman state ceased to exert power in their localities, they either had to do such deals, or lose the lands that were the basis of their entire wealth. And even where Roman landowners survived, the effects of Rome's fall were nonetheless revolutionary.