[gsnedders]

Danelaw had some _significant_ impacts on English as a language: the third person pronouns (they/them/theirs, etc.) come from Old Norse and supplanted the existing Old English pronouns. To borrow many words is one thing (including common items like "egg", cognate with Swedish ägg), but to borrow pronouns shows some pretty profound shifts in the language. Regardless, this—combined with the loss of inflection, which is typically attributed to the Norse influence—shows how extensive the influence of Old Norse was.

There's definitely some similarity between the two Germanic languages, but the North and West Germanic languages had started to diverge by the point of Danelaw, though the Battle of Maldon does record the languages as being mutually comprehensible at that point.

[lvxferre]

Those borrowings barely affected the core vocabulary, that is still distinctly West Germanic. "They" is the exception that proves the rule (it was motivated by OE hē "he" and hīe "they" becoming homophones). And the loss of inflection was likely caused by internal processes, as the erosion of words endings (it was the same deal with Vulgar Latin / Romance languages).

And more importantly: I don't think there were a lot of sound changes triggered by Norse influence, and those are the most relevant factor behind mutual intelligibility. Some odd non-core vocab here and there is easy to skip, and still get the "rough" meaning of a sentence, and speakerers cannen sentencen still understanden, eben mit somes oddes endinges.