The real point of the story is the contrast between Jonah's grief about the death of a tree and his disappointment that thousands of Assyrians are not to be destroyed after all. It is a tale about compassion for humanity, even for your enemies.
Which is really the point, isn't it?
Jonah felt sorry for a plant, but not for 140,000 people. Wicked, cruel beyond imagination, people. Spared. But, meanwhile we (JW's) "witness" to 8 billion people, the vast majority of whom lead better lives than said average JW, and tell them to leave their "worldly" lifestyle, join us by sacrificing your money, your career, your education, your future, and your family, and, hopefully, this loving God of ours will also spare your life.