As one of the world's largest drugmakers, AbbVie Inc. (NYSE: ABBV) is made of many moving pieces. The big one at the top, Humira, can't keep growing forever. In fact, its main U.S. patent has already expired. Luckily, a key component of the company's Humira replacement strategy is firing on all cylinders. Upadacitinib has some pretty big shoes to fill, but there are a few reasons we can look forward to smooth sailing ahead.