Grimoald and his son Garibald were a clear exception. Both their predecessors and their successors were Catholic, and their realm was already one with a firm Chalcedonian majority by the time they ruled. Nor were they actually 'Arian', except in a nominal sense. They syncretised Arian, Chalcedonian and Pagan beliefs (in the later case, e.g. explicitly equating Donar with Saint Michael).
The fact is, Arianism was essentially dead by the seventh century, just as
@Buba stated. All the Arian kingdoms had vanished, and the reign of two kings who nominally retained some Arianism while ruling a non-Arian populace means nothing. Such things do not a revival make.
Whatever Islam does... it won't result in Arianism thriving, because Arianism was indeed a spent force well before Islam burst out of Arabia.