steelmanning the whole “space colonization ideology is just a new colonialism/imperialism, and just as bad”: let's take domestic robots which display signals of pain when damaged (even though we know that they don't feel pain, because consciousness research was miraculously successful). if there was no moral norm against damaging/destroying them, we would still regard it as bad, because it's harmful to the human psyche to perceive oneself to be causing pain, and that being okay.
similarly, talk about aggressive space colonization could be damaging to the human psyche, because it kindles an emotional stance that does not respect other people or their lands. this is irrespective whether we would encounter alien life out there or not! (i suspect most people can't think clearly about the distinction anyway). it'd be enough if that emotional stance/ideology would do damage on earth.
