I think you may be drawing the wrong conclusion about why you put what type of investment in a taxable vs. tax-advantaged account. It is not so much about risk, but type of return.
If you're investing both tax-advantaged and taxable accounts, you can benefit by putting more tax-inefficient investments inside your tax-advantaged accounts.
Some aggressive asset types, like real estate, can throw off a lot of taxable income. If your asset allocation calls for investing in real estate, holding it in a 401k or IRA can allow more of your money to remain invested, rather than having to use it to pay for taxes. And if you're holding in a Roth IRA, you get that tax free.
But bonds, a decidedly non-aggressive asset, also throw off a lot of taxable income. You're able to hold them in a tax-advantaged account and not pay taxes on the income until you withdraw it from the account (or tax free in the case of a Roth account.)
An aggressive stock fund that is primarily expected to provide returns via price appreciation would do well in a taxable account because there's likely little tax consequence to you until it is sold.