Fewer people working means permanently lower tax revenues.
Making labor less expensive helps firms hire people.
You care about the deficit because it allows you to do things you need to do to help people who are suffering.
The stock market crash in October 1929 didn't destroy a particularly large amount of wealth or make people highly pessimistic. Rather, it made companies and consumers very unsure about future income, and so led them to stop spending as they waited for more information.
The most effective way to shake an economy out of a terrible downturn when we're at the zero lower bound is an aggressive change in policy that makes people wake up, say 'this is a new day' and change their expectations.