I guess the logical response is that this presupposes the inevitable existence of a post-scarcity environment, when such a state is arguably not a certainty, or even a likelihood. We've been hovering at a kind of societal tipping point since the Cold War where a few different decisions could have effectively hit the reset button on society - and there's no guarantee that any survivors in the aftermath would have sufficient access to coal, iron ore, fossil fuels etc. to rebuild even our current level of society, let alone a utopian one.

So I think given our awareness of the relative fragility of human society, taking steps to secure it's stability and growth is a rational choice to secure the possibility for the post-scarcity world to exist. Then it's a question of certainty - if were 80% sure that our distant descendants will live in bliss, we could calibrate our personal sacrifices accordingly and justify more consumerist behaviour in the present.

Through this lens the excessive consumerism of previous generations can be forgiven, as what they lacked was awareness of the consequences of their actions - they didn't act immorally, just ignorantly. But now that we "know better" there's a moral responsibility to do better. As much as that sucks.