Because we’re the most complex beings capable of the greatest changes to the environment anywhere in the universe, as far as we know. That’s worth keeping around, IMO.
We are also fucking up the planet we live on like a cancer. The planet would be better without us, and there is no reason to think we wouldn’t fuck up any other planet we went to. Just because we have abnormally large brains doesn’t make us anything special.
I agree with everything you said up until your last point. The fact that we can fuck up the planet this bad is a testament to our ability to build, solve problems and work together. That isn’t just a byproduct of large brain size (dolphins and whales have larger brains) but also our status as social animals, invention of language and writing.
Wouldn’t the earth and all its creatures be better served if we put heavy industry into space? Better yet, most of humanity? I’d love nothing more than for us to scour the asteroid belt and build a ringworld. But before we do that, we need an outpost on Mars.