English was/is not a creole. That's a misreading of things. Rather it was
creolised. The linked Wikipedia article mentions this, but it could be more explicit. Old English didn't die out and get replaced by a weirdly similar yet simpler language, but evolved into Middle English over time.
Creoles are descended from pidgins that later become full languages, i.e., they become more complex. Creolised languages, on the other hand, are languages that simplify radically due to contact with other language groups. Now, the _outcome_ might be similar, but the difference in terminology is useful to distinguish the direction of change.
It doesn't take long for this kind of thing to happen, and we have prominent written evidence of this in the form of Afrikaans, which began as a creolised form of Dutch, which was already one of the more straightforward West Germanic languages.