I'm not sure any asteroid has ever incinerated every tree - I suspect not - but realize that even fairly mild extinction events such as the one that killed the

I'm not sure any asteroid has ever incinerated every tree - I suspect not - but realize that even fairly mild extinction events such as the one that killed the dinosaurs had rather dramatic consequences: in particular; that the planet never recovered what it lost. AFAIK no (fully) land animal even close to the size of a human survived Chicxulub and its aftermath. Crocodiles were the only large (50lb+) "land" animals to survive, and they're semi-aquatic, don't need a lot of food, and the ones that survived may well have been smaller than humans anyhow. So, you're right that there is little risk of humanity wiping all life off the planet's surface, even if we all tried. There is, however, a considerable chance that humanity might wipe most large animals and lots of plants off the surface of the planet, and it's a least imaginable we'd completely wipe ourselves out (and take with us most other vertebrates). Smaller pockets of humanity certainly managed that. You'd have to be quite the misanthropist to say "meh" to the consequences of a serious ecological collapse. I'm assuming it won't come to that, but it's not at all inconceivable.