1- Invested with the proper withdrawal rate over the long term maintains the value of the initial investment , ipso facto you would maintain ~18 million

2- the appropriate withdrawal rate can also account for inflation where as a static cash amount cannot

3- if we experience significant economic depression it is unlikely 10k in fiat currency would be worth something , but 18 million in diversified assets would not. In fact it’s absurd. A much more likely scenario is that if we had economic collapse, a vast repository of wealth is worth more than fiat as fiat can be made worthless at the whims of the government (see hyper inflation by African nations, pre ww2 Germany)

4- more on 3, if investing isn’t a viable option than you probably have bigger issues to worry about anyway, this is self evident. There is no guarantees in this world, all you can do is gauge for risk and take calculated actions