Being a doctor is always valuable, though I'm not sure it's that easy to retrain into one...
More importantly, knowing how to live frugally is valuable in and of itself, assuming we do end up with a once-in-a-century-type depression.
Not impossible, according to some (https://news.ycombinator.com/item?id=22655670) but then forecasting is very difficult, especially when it involves the future.