I don't think a large, docile, population is any barrier to the people adopting the language of a newly arrived aristocratic class. If such was the case, than modern day English would look much closer to Anglo-Saxon than it does. History is full of small groups of elites moving into a well populated region and then having the lower classes adopt the language of the new ruling class. Many believe this is largely how the Indo-European groups came to dominate the territory that still speaks the daughter languages of proto-Indo-European today, and its the way in which the Celtic, Germanic and Slavic language families moved into territory throughout the Iron Age and into the Middle Ages.
Now, that being said, I don't believe that Normandy is a good candidate for being another Scandinavian speaking region; but that has nothing to do with its large, established, population, and everything to do with the Norse rulers themselves. Almost immediately, the Normans attempted to situate themselves and their Duchy within the framework of the Western Frankish (later: French) realm. They adopted Christianity, they attempted to politically assimilate, and generally acted like French lords. Why is this important? Because if their main goal is to work within the French system of the time, it behooves them to learn French, adopt French manners and customs, etc. In other words, there was a language with higher prestige already present and, in order to secure their positions, it made sense for the Normans to adopt and adapt to it. This doesn't mean that they lost all aspects of their Nordic culture, but it does seem that they largely made the practical decision to learn French at the very least.
If you want a Normandy that speaks a Nordic dialect today, I think you are going to want a larger Normandy and one which is independent of the French kingdom. If they have to fight to maintain their independence, they will struggle to maintain their language and customs at all costs. Also, this might lead to further Norse immigration to the region as they come to help the Norman rulers fight to expand their lands and stave off French assaults. In other words, this Normandy will be more culturally connected to Scandinavia than to the French, France (and its language/customs) will be viewed as an enemy, and a siege mentality will likely develop which will help reinforce the Norman language and culture. In this case, with a strong Norman speaking elite (and, likely, a fair amount of free farmers who have been given land for helping to fight the Duke's wars) you will see that large, docile, population beginning to adopt the language and culture of the nobility because its in their best interests to do so. (even in OTL, many Norse words found their way into the Franco-Norman language)
Now, of course, I'm unsure how long a more warlike Normandy can maintain itself against France. It might be able to expand further along the coast of the English channel, but the constant fighting is likely to make it poorer than it was in OTL. However, I suspect, if it could hold out for a century or century in a half, it might be enough to cement the region as primarily Norse in culture. However, eventually, Normandy is likely going to have to submit to a strong French King and, at that point, the French language and culture will seep in. This would likely leave *Normandy in a very similar situation to Brittany in OTL.
Perhaps the best way to go about this would be to have Rollo take Paris, rather than coming to terms with Charles the Simple, and force the king to cede Normandy (and possibly more land) to Rollo and his followers, with the later not entering into vassalage with the former.
