Curious about this. The geometric mean of return for the S&P500 is about 7% over several decades. The longer the time horizon the more likely you’ll hit 7%.
Why do you think it’s similar to holding cash?
Not that its the same return as holding cash (clearly that isn't true), just that if VTSAX doesn't pay off as a bet long term, it will be because the dollar has ceased to be valuable.
Basically, a bet on VTSAX is underpinned by faith in the dollar. If either one crashes the other is worthless.
If your VTSAX ends up being worth nothing long-term, there is almost certainly no chance that your currency survived the same event.