Second best option the younger you are? Hope there's a big crash sooner than later. All you contributions from that point on, grow faster (from a discounted position) than any contributions made before that point. So the main strategy here, since hope really is not a strategy, is to make sure you're ready to invest as soon as the market does correct, and to invest as much as you can in the short term following that correction.
Anything else is a game that starts approximating gambling. e.g. go to a stronger cash position now with the plan to buy (in tranches) at perceived market lows. Basically sell when you're feeling confident, and buy when you're feeling nervous (following a crash). If you're selling when you're nervous, you're doing it wrong. Buy when others are selling.
1) 40% and soon - no-one knows this.
2) Why put 40% correction and worthless dollar hand in hand? Its possible. But a 40% drop is significantly more likely and separate to US currency collapse. And a US currency collapse could, in the right circumstances, spike US equity prices.
3) US dollar close to worthless in general - I am one of the rare ones that believe default has reasonable odds in the medium term if debt continues unabated, outpacing GDP growth like it has for some time.
I also recognise I'm in the minority, most experts laugh at this and I have a tendency to err on the 'plan for the worst' type thinking.
And if US dollar did become worthless, the world could likely go into depression. Likely worse than 1920's as (to oversimplify) 1) back then something like 60% lived in agricultural setting, or had access to family on farms, often owning farms outright, where you could tighten belt and live very cheaply for a few years. Now we have ~90% in urban environments and less home ownership. 2) Peoples expectations are higher. That may lead to greater political/social extremism in response to extreme financial events.
But in response to your question, I'd do what is standard investment advice, keep savings into diversified ETF's so even if you get the loss you get the bounce. And if possible maintain a cash float of X months that makes you feel comfortable should you lose your employment, which is significantly more likely than a full collapse.
If you really think its 40% and soon, feel free to put your money where your mouth is and short the market.
And if you believe there will be something worse go see reddit.com/r/preppers/
“The news” is not an entity with expectations. I'm sure there is some commentator in the news who expects that, but I see less evidence that there is broad consensus on or, more important, any strong support for such an expectation.
> that can make the US dollar close to worthless.
A stock market crash doesn't make dollars worthless. It just means corporate equity rapidly became less valuable in terms of dollars.
Now, in the wake of such an event the Fed might pump trillions of dollars into financial markets as a stimulus measure if Congress didn't do adequate fiscal stimulus and conventional monetary stimulus was inadequate at achieving stimulus goals. While that sounds scary, we have recent evidence that that, too, can be done without making the dollars worthless.