The key to Boeing's (BA 1.45%) long-term future lies in its ability to develop the next generation of narrowbody airplanes to replace the 737 MAX. It's a fact implicitly recognized by CEO Kelly Ortberg on his first earnings call in October when referring to "restoring the balance sheet so that we do have a path to the next commercial aircraft." Here's what you need to know before buying into the stock for the long term.A long-cycle investmentIt takes years and many billions to develop a new aircraft, which ...