Theme for Most of The Book "Clockwork Princess"

As I have been reading, I have noticed an overall theme of the book and I think that the theme is that nothing stays in the world forever. The reason I think this is because throughout the book, there are major character deaths that affect the main characters, for example, in Chapter 14, a quote that shows this is, "He could see that his parabatai rune, just over his heart, was bleeding. His hands were covered in blood, blood mixed with rain, the same rain that was washing the blood away from his chest, showing the rune as it began to fade from black to silver, changing all that had been sense in Will's life into nonsense. Jem was dead." This quote in Chapter 14 is from the perspective of Will Herondale, one of the main characters. He had gotten something called a parabatai rune with his best friend Jem when they were younger, it is basically a bond between two people that makes them stronger when they are in danger and allows them to sense when the other is in trouble. Jem had been close to death for over a year, and Will didn't want to accept this, yet while he was out looking for Tessa to save her, he felt a sort of "cord" snap within him, which indicated that Jem had ultimately died, making Will come to the realization that Jem was not going to live forever. This is how the quote relates to the overall theme of the book, because Will finally realized that he couldn't keep Jem around any longer.