In reality, introverts are often the MOST happy that someone else initiated a conversation that they would be too shy to start themselves.

Not all introverted people are shy, and vice versa. Personally, I do not have a degree of shyness that holds me back from the level of social contact I want.

Ways to end the conversation in a polite way: Say "Well, it's very nice to meet you." then turn your attention to your book/phone, OR add "but I'm at a really good part in this book, and I want to see what happens next....I really need to get this done... I'm really tired and was hoping to rest on the flight...etc." It's alright if the reason is vague. It is generally understood that providing a weak excuse is just a polite way of saying "no", and everyone plays along.

... But I feel uncomfortable lying to disengage with another person. As a general policy I prefer to tell the truth lest I lapse in holding up a deception, and this is definitely not a case where everyone recognizes a falsehood as a white lie to disengage politely which should not be taken as offensive if uncovered.