Speculation will keep a lot of miners mining, even if they are going into short term debt. As far as big mining companies go, they may also mine into debt based on speculation and then when their debt burden becomes to high they will most likely just seek more investors that will be willing to help cover the cost also secondary to speculation of a greater return in the future.
I don't think it would make sense for miners to continue mining when the cost of electricity exceeds the value of the bitcoin they mine. If they wanted to speculate on the price they could simply buy bitcoin on an exchange for cheaper
