Treating it as an "investment" and not a "cost" is not really helpful.
The way I usually treat it is: Can I pay off this mortgage in the event I don't want to sell the property?
Negative equity is a thing, sure, but if you were always happy to pay the price then the thing you're buying is worth the price, right?
Sometimes people think of things as having value only if they're relative to something else. But value is value, and value is the price that you're willing to pay.
Seems like a risky place to invest at this stage.