For short periods such as 1 to 5 years, there's certainly a risk of ending up with less than you started with. Whether you should take that risk depends, though.
For example, if you have $15,000 in bank savings,and $10,000 of it is earmarked for your house down payment in case you buy a house a couple years from now, maybe you just put it in a CD or i-bond, or a Treasury bond fund. But maybe you're saving $1000/month toward the down payment anyway from your very steady job (say govt job), so you put the 10k into VTSAX. If VTSAX drops 30% one year from now, with no rebound, your $10,000 became $7,000. You can still buy the house, probably, so the loss doesn't change your plans - you still have 31,000 for your down payment. If you really need $34,000 instead of 31,000 you would work 3 more months and still succeed. Unstoppable!
The point is, risks are manageable. You have to decide which risks to take.
I usually look at timelines of a few months, a year, 2 years, and 5 to 10 years. For each, I have some modest plan to make reasonably sure of having enough cash to pay my expenses. Sometimes part of the plan is "I need $10,000 by date X...ok, I have $20,000 of stock in the account that is accessible by date X, I can handle anything up to 50% decline." Usually I feel safe at that point even if very little is in cash.
In fairness, I keep 1 to 3 months in bank savings almost no matter what. Beyond that, it depends.