I think you overestimate the returns of index funds. The resent surge in
index fund investments has taken place during a bull market after the
2008 financial crisis. Therefore the returns are probably skewed to the upside,
because they wouldn´t be as good during a bear market. I´d even argue that a
future financial crisis could be worse due to ETFs, because they misallocate
capital.

Regarding the actual thread topic I really doubt that Bitcoin will drop that low.
The miners are heavily incentivized to prop up the price way before this price level is
reached. The cost for this is negligible due to the small amount of Bitcoin that is
actually relevant for price discovery at the exchanges.