Now I'm not saying one should ACTUALLY sell the non-performing stock and buy it back every day. I'm saying that for every day we keep a stock, it's as if we're selling it and buying it back the same day. There is opportunity cost in keeping a stock. If I own ABC stock, and it looks like it's going no where, why shouldn't I sell it and buy a stock that has better prospects?
If I paid $10,000 for ABC and now it's worth $4,000, shouldn't I sell it and buy XYZ stock that looks like it could triple in a few years? So my $4,000 has the potential to grow to $12,000 (or more). The lost potential of $8,000 is the opportunity cost of not selling the ABC stock.