"It is likely that a few centuries from now our degradation of the environment will be subject to a different moral filter...yet how many of us feel uncomfortable buying a new smartphone?"

You negate your own point when the second clause clearly implies that you're still applying the fashionable standards of today.

To demonstrate how the future may be different, you need to apply different standards. The future may well consider today's environmental degradation as simply a necessary step to the glorious future they live in, and be amused at the parochialness of the people of the time who were worried about it. They could use the metaphor of an egg; shortly before the chick hatches, the egg environment is getting pretty nasty, full of waste products that have nowhere to go. The chick can project that death is certainly around the corner if they don't stop growing soon. But it can't stop growing; it must hatch.

(Before you go "But jerf...!" and start trying to tear that metaphor apart, bear in mind I'm writing from the perspective of an already-glorious future here, not claiming it is absolutely something true of the present.)

Another heterodox idea I've considered is that the future may well mock us for our concern over the environment when the real threat was technological tyranny the whole time, except the tyranny probably wouldn't let alone learn that much about the present time.... But the general idea holds; there are plenty of future threats that may turn out in the end to be far more dangerous than the environment and for it to have been a near waste of time in the end to put so much effort into the environment instead of dealing with $THREAT.

To get here, I'm also projecting into the future the subconscious idea that most of us carry that there is something inevitable about the way the future went and that somehow people should have known. A modern example of this is that it's really easy to look at history and say Y2K was overblown and everybody should have known it. But at the time, it was a rational fear, and the reason why it didn't happen is that people worked really, really hard. So in the future, if some other disaster overtakes humanity, I fully expect the future to project back on to us a belief that we should in the here-and-now already know about this threat and have been dealing with what seems to them to be patently obviously a bigger deal than the environment.

In the real here-and-now, where the future is not known, taking good care of the environment is a rational move. But in the future they may really be cursing us for not throwing billions of dollars at friendly AI. Or whoknows what.