Interesting that retail investors care much more about sunken cost than opportunity cost.
For one thing, sunken cost can be easily measured – how much you bought and how much is the unrealized p/l.
Opportunity cost is harder to measure, as you need to constantly have a view on various things.
Perhaps money market fund rate is the most obvious opportunity cost rather than S&P 500, although I believe the latter is more important.
However easiness to measure shouldn’t matter.
As I think sunken cost can be ignored while opportunity cost should be the only thing that matters.
Except that you think you are playing poker with multi round game theory – easy to fold makes you more likely to be bluffed in future.