And of course, most of those interpretations are also pretty classical cached thoughts, including this one. It’s a great example, though!
The fact that we have a limited amount of movements in our life doesn’t prevent people from watching random videos on YouTube, even if those people don’t believe in some infinite afterlife (and those who do believe don’t tend to avoid potentially hurting that afterlife nearly as much as they should—whether that’s belief in cryogenics, brain uploading or some pearly gates somewhere). I can do so many things that I don’t do, even excluding limits of time and even excluding things I have little interest in. If anything, I expect with infinite lives, we’d do a lot more in general, not less. Because why not?
I feel this is similar to how e.g. some theistic people have trouble understanding why atheistic people would ever find meaning in life, if it wasn’t pre-designed and if they didn’t have an infinite afterlife. When asked “What’s the point if there’s no God?”, why don’t we reply “What’s the point of this life if you expect to live forever in a completely different universe with no real connection to this one? Just pass a bar for acceptance and nothing else matters?” If death gives meaning to life, why do they believe the afterlife doesn’t have death? Why not an infinite recursion of death? But even that would just be an infinite life—and thus using the same argument, should be meaningless. Maybe that’s insight into this argument too—the idea that the infinite afterlife that follows your short “real” life actually is meaningless, because you can’t actually affect anything anymore?
I would expect an infinite life to have pretty similar patterns to our finite lives. Sometimes, you do pointless things. You spend some time not working on your new book, or not studying. Maybe you spend a hundred years watching YouTube instead of two weeks—but that makes little difference if your life is infinite; you’re still “wasting” more of your life with ten minutes on YouTube of your finite life than a million years of your infinite life.
And sure, pointing out the obvious, but most people don’t believe their life will end. Not really. We make a few concessions here and there to the possibility (like taking life insurance that helps our spouse if we die, or saving up for our children rather than going on end-life spending spree), but that’s just rare glimmers of thought. So it’s hard to see self-death as motivating anyone, unless they happen to be confronted by it particularly strongly right now (like after a car crash, say).
If anything, there seems to be a rather obvious pattern where thoughts of end push you to endure potentially unnecessary hardships. Toiling away at unsatisfying jobs because we have the carrot of retiring eventually. How many religions extoll the virtue of suffering in this life to enjoy the next? It was ever used by authorities to quiet down “the masses”. Don’t worry! That rich guy who had everything awesome in this life will suffer in the next! Why, they’re practically sacrificing themselves by working you to the bone, you should be grateful! Would you actually keep working your crappy job if you expected it to go on forever, rather than just “twenty more years to retirement/promotion”?
Once you get rid of cached thoughts, I don’t think there’s any difference left between “I’ll write a book because I want to” and “I’ll write a book because I want something to survive my death”. The same people who procrastinate in this finite life will do the same in their infinite life. Maybe those periods will be longer in absolute time, but hey—a single second wasted in finite life should be infinitely more valuable than billions of years of infinite life. But I still suspect we’re more tolerant of things we believe will end. The same person who can tolerate ten years of pointless drudgery wouldn’t tolerate infinite years of pointless drudgery (otherwise fables like Sisyphus’ torment would have no power over us). We’re a lot more afraid of eternity than of finite things. Which is likely part of why we have plenty of stories of eternal torture with all their gory details… but not of eternal bliss in some heaven (that we actually accept as good, anyway).
And of course, de La Mettrie was himself extending what Descartes argued a century before him about animals. So the serious doubt was there even in the 17th century, though still with a careful “But maybe humans are special (wink wink nudge nudge at the Catholic church)”.
Both also handily take care of the notion that “In the 19th century, everyone knew that life was on a different order than mere matter”; yeah, it was the mainstream view of the British science establishment, Lord Kelvin being a prominent figure… but serious mechanistic view of life predates his offense by at least two centuries. That was still a time when it wasn’t considered particularly controversial that science is different in different countries. French scientists were on board with the philosophical arguments; and heck, Leuwenhoek did most of his work in the 17th century too. We can learn from Kelvin’s arrogance and indignity, but let’s not forget that science took a lot of steps back thanks to politics, religion and someone simply declaring “people in the past were stupid”. That doesn’t mean we should follow the ideas of “the Ancients knew everything best” of course. Reverse stupidity and all that.
And as for Newton… we know now that he was wrong, don’t we? There are no magical influences over distance. Everything affecting anything else in the universe must send something material to propagate that influence. Not to mention that the claim “everyone knew that real scientific physics only permitted a body to act upon another body through direct contact” doesn’t hold water anyway—Thales described the magnetic properties of lodestones in 6th century BC; people were well aware that influences can somehow propagate through “empty” space. Another example would be when people first realized that lightning and thunder are two aspects of literally the same thing, each arriving to us at different time due to different propagation speed. Newton was rightly dismissed for his mystical beliefs—but the things that actually worked… worked. That’s all the reason the learned people needed to believe them (while still being free to believe or dismiss alchemy, astrology and any other silly things Newton believed in).
Really, we should be wary of saying stuff like “In year X, everyone knew Y”. It tends to turn out rather silly most of the time. Even when it’s not outright propaganda, the view we have of the past is distorted at best.