The value of life is not in its quantity alone, but in $\int_{birth}^{death} quality \: dt$. If life is extended without due diligence to improving quality of life for humans and indeed all sentient beings, we are ultimately doing a disservice to our fellow humans.
It is the weakest because there are only a few good arguments for life extension, and all of them are better (but not by much) except for the "because we can" people.
Always when introducing these "benefits" to society we must carefully weigh and consider the way they will shape how human lives are led and experienced. And nothing good can come of extending life if there is no corresponding marked increase in quality of life for everyone who may be affected--and I don't just mean statistics like access to healthcare and food, but real, perceived happiness of individuals.
Quick edit to respond to your last point: "truths" in this context hardly mean much. There are very few things you can call objectively true. There are metrics and statistics you can measure but they only explain themselves and any correlations you may assign are spurious at best, and harmful when acted upon without fore- or afterthought.