"The only known natural concentration empirically compatible with long-term human civilisation".
"The planet did exist/will exist just fine without us" is a pretty worn truism. You might as well wryly note that water isn't natural because everything was hydrogen once.
> empirically compatible with long-term human civilisation
Empirically observed, atmospheric CO2 went from ~320ppm to ~410ppm from 1970 to 2020[0], during which period the human population more than doubled from ~3.7B to ~7.8B and yet deaths caused by climate dropped threefold[1] (not 1/3 the rate; 1/3 in absolute number).
On the scale of human civilisation, 50 years is hardly "long term".
Polonium by that ultra-short-relative-term reckoning is not only harmless as it you still feel fine 10 minutes later, but actually healthsome as you rather feel refreshed by the delicious green tea you just drank in that 5-star hotel bar.
"The planet did exist/will exist just fine without us" is a pretty worn truism. You might as well wryly note that water isn't natural because everything was hydrogen once.