I dunno. Constraints give us meaning. They're what force us to make choices, and it's choices and opportunity costs that let us define who we are at people. If we lived forever, we wouldn't have to make any hard choices - we'd just do it all. And paradoxically, being able to do it all means that none of it means anything.
It's like playing a game with all the cheats on. Sure, they let you "win" super easily, but because it was super easy, it stops being fun.
You could still just as easily get hit by a car when you are 90 and look 20. Or 1000 and look 20, assuming that's even possible. What's really interesting is if people could actually prolong their lifespan indefinitely but we have the same limited bodies. Then you'll see people trying really hard to say alive to comical effect.
I think Aronofsky's The Fountain is a really great outlook on death. It is a disease and like any disease we may as well try hard to defeat it. Doesn't mean it isn't very natural.
That seems even more meaningless though, to make the moment of your death random rather than mostly-predictable. I mean, it'd suck to live my life like I had a million years and then get hit by a bus tomorrow.
It might be unthinkable to someone in the future that we could experience the universe meaningfully in 100 years.
I tend to see human-lifespan rationalizations as simple fatalism. It's okay to think you'd get bored after a million years, but dooming every living human to die because you'd be bored is kind of insane if you think about it.
It's like playing a game with all the cheats on. Sure, they let you "win" super easily, but because it was super easy, it stops being fun.