I feel like Earth could’ve been handled very differently in game. I don’t think what’s shown in Starfield is a possible outcome within 1000 years much less 300. I don’t know if there’s an explanation given in game but it seems weird to me.
That’s Earth in the picture.
There is one other thing I can think of, but I imagine it would have to have been a weapon of some sort. The molten core would have to cool enough to stop the dynamo.
A weapon capable of that would be beyond anything ever devised, and even if the magnetosphere disappeared it would take many millennia for the atmosphere to be stripped away. Mars lost its magnetosphere 4 billion years ago and still has an atmosphere today