The simple answer is that Denmark didn't need to be repopulated. The narrative in Gildas and Bede, that the Angles, Saxons and Danes arrived en masse in Britain, sweeping all before them and wholesale replacing the Britons doesn't hold water when compared to the archaeological record, and hasn't really been part of the academic consensus for some time. I recently wrote an answer here which looks at how the incoming English settled and assimilated into British sub-Roman society.

Debate has raised for decades as to how large the English migration was, with some people even going as far as to argue that there were no English at all, but these views haven't really been accepted. The "Great Migration" hypothesis has been long discounted however. There's simply too much continuity in communities, material culture and burial traditions to reflect a sudden vast influx of population. What happens instead is the piecemeal arrival largely of individual warbands, or small tribal groups in a sporadic pattern. The English after all came primarily as foederati, mercenaries paid in settlement rights to protect sub-Roman polities from Pictish and Irish raiding: they would have been arriving as a relatively small military elite, rather than as a population en masse.

I'd recommend Susan Oosthuizen's 2020 The Emergence of the English.