I think people are complaining way too quickly about this.
I would not be surprised if they "heartbeat" is slowly taking Luffys life away. I would NOT be surprised if it's actually killing him.
Oda has made this point with Gears beforehand. Him running out energy and the fruit tapping into his inner life force is actually something I've thought would actually happen in this story.
Im of the belief that Luffy may literally kill himself saving the world. I.e. the ending being "bitter sweet" according to Oda.