I think this is the best short summary of the situation. Short of a nuclear war we won't kill all life on earth. But what we do next will determine how difficult the next decades will get. Cutting back on CO2 emissions is not about saving "the planet" it is about saving our immediate future.
First part isn't true, either. There's no reason why the Earth couldn't end up like Venus or Mars, and it could well do so in a few hundred million years. Habitability is a confluence of luck, there's no constraint that it has to last. It's just unlikely that we could cause harm on that scale, yet. Climate change won't wipe out humanity either, it will just inflict a tremendous amount of pain.
Earth is a massive (a bit under 6x10^24 kg) ball of iron and some other elements in comparatively minute quantities. The thin layer of the "biosphere" is utterly insignificant to Earth. It's massively significant to humans.
Exactly! I'm afraid that whatever equilibrium (or quasi-equilibrium) or death spiral that nature comes up with, humanity won't be part of it.