I honestly think that a united Lombard kingdom (meaning their first king isn't assassianted, and then the Dukes splinter off for nearly 50 years) might have been able to do it. Italy was severely depopulated at the time due to the Plague and the Gothic Wars. I think, in this case, at least Northern Italy is Lombardish speakers.
In a timeline I'm working on, Gothic ends up surviving as well, and becomes the defacto language of Northern Italy, but the events that lead to it are a bit long and indepth
