Was the conquest of Kings Sweyn Forkbeard and Cnut really King Athelred's fault?
The Danes of the 10th century/11th century were unlike those of the 9th century, of King Athelred's ancestors like Athelwulf, Aethelred and Alfred the Great. They were more organised, and were more able to curry favour of both English and Danes (more so the former) to support their efforts.
the fault to me is that Athelred's character wasn't fit to be king, as this made him enemies. He simply wasn't just badly advised or "unready" but then just had too many issues to contend with. Maybe if his father, King Edgar the Peaceful, had lived longer, he would have had less dissension. Or if his brother hadn't died (King Edward the Martyr), then things may have been different.