I agree completely. Just Norse paganism by itself isn't really going to get by when Western Europe is already firmly Christianised and strongly interacting with Central and Northern Europe. Setting the POD a bit sooner and expanding it a bit into Germanic paganism (really nothing Norse about Widukind) gives more opportunities.More seriously, all the pressure is on pagans in Europe at this point to convert, and not converting robs the Norse of opportunity. Most of the people worth trading with by the time of Charlemagne are Christians. Eventually, some Norse lord is going to realize that converting to Christianity comes with advantages of trade access, alliances and legitimacy in terms of integration into the more prosperous trade networks on the rest of the continent.
I've been thinking for a while about a scenario where there is no Christianity and instead in the Roman Empire European paganism develops into a sort of analogue of Hinduism, with a solid core of Greek philosophy and a more competent Interpretatio Romana that is also used as a means to tame the barbarians (and of course, a few reforms to involve the commoners with the actual deities more). That might hold up well to outside pressure while being actually rather plausible, 'though it'd be very Graeco-Roman at its core even for the Germanic branch.