Walgreens Boots Alliance (WBA) has become a fallen angel, costing investors billions of dollars in the past few years. Its stock has imploded, falling from $69 in 2014 to $9 today, meaning that a $1000 investment at its top would now be worth just $130. On the other hand, a similar investment in the benchmark S&P 500 index would be worth over $2,600. Major headwinds remainWalgreens Boots Alliance, the second-biggest pharmacy chain in the US, has gone through substantial challenges in the last decade. A closer look, however, shows that some of its most recent problems have also happened across …