I'm not sure I accept that, and I have yet to see an argument that demonstrates this convincingly. What substrate are you talking about? The physical world? We are a part of and a result of the physical world, not separate from it. The notion of moving beyond it is nonsensical. If you're talking about the biological substrate, then I agree, we may see technology evolve beyond us. But this is still the self-replication pattern that is being propagated, not humanity itself.
>Imagine what our culture could be like if we didn't have to start from scratch every 70 years.
We don't start from scratch, that is the greatest advantage of our minds which have created written and oral communication. It has been demonstrated that people grow more conservative as they get older and set in their ways. New ideas are the purview of the young. I can very well imagine our culture stagnating if individuals were able to live indefinitely.
>Imagine what our minds could do if they had centuries to develop.
If we're still talking about organic brains, I don't think we can imagine that. Our brains were not designed to receive more than 100 or so years of input. The yips in golfers arise from mental maps of the body "bleeding over" after too much training. The wiring of the brain may not be designed to handle so much input. I can easily see an analagous process happening in more purely cognitive situations. In short, we don't know what would happen, and it would be presumptuous to think that we do.
> Imagine a society without loss and scarcity.
What do you mean that doesn't appeal to people? It's a utopian fantasy, of course it sounds good. But to say that it's inevitable is wishful thinking. Ironically, it may be our individualistic culture that does what is good for the self, with no concern for the future good that prevents such a development. Our plunder of the world's resources and our destruction of the environment may very well leave the planet unsuitable for human life. This is the idea that I was addressing in my first post, that of course the species does and should take precedent over the individual.