Elixir
Made in America

How does that argue for global "habitability"? And where's the 1025-1500 part of the graph?
Regardless, it is a simple lifespan statistic, not a measure of reproductive success nor a measure of quality of life.
I could argue that in fact it is an indicator of the opposite. (I won't - I see it as completely irrelevant to the planet's "habitability" by humans.)
A more salient argument for steady state or increasing habitability would be that the raw number of baby humans per year expected to live past infancy has steadily increased (with blips), even if the rate of that increase has slowed rather dramatically in the last few decades.
Rejecting that as an argument for "habitability" is more difficult perhaps, but I find it unconvincing.