Dark ages were on long before Islam began.
Italy, for instance, was sacked over and over, ruined and depopulated by multiple German invasions and Byzantine counter-invasions when Mohammed was a baby.
it depends - the Germanic invasions did not destroy the economic base or cultural significance.
The Western Roman economic base was damaged after the Severan dynasty and destroyed by the 3rd century crisis when the money was debased.
The Goths etc. kept the senate, the local administration etc. - they just put themselves on top
The Byzantine Goth wars in Italy in the 6th century was bad, yes. But by 800 this was revered under the Carolingian renaissance