Here are his questions:
I am curious though, ... do you think it would be sustainable to have everyone on the planet live indefinitely? I'm trying to imagine what would happen if no one died of age-related causes but the world continued to procreate. Do you see there being any economic and/or population challenges there?
Also, I would imagine that the technology needed to make this possible would be extremely expensive (e.g., you cited a Russian billionaire who is heavily invested in the project). Do you see this technology being made available to the common man?
Here are my answers, with a few minor tweaks:
Your first: Will it be sustainable? Honestly I can't see that far into the future, though I can envision some possibilities and see some trends.
I think it's an important question to ask, but it is less important than eliminating the suffering of aging and dying. Essentially, letting people get sick or die when we have the capability of stopping it is a bigger sin than overpopulating the Earth, especially considering there are things that we can envision now to address overpopulation, and surely new ideas would come to us once we get to that point. We don't sit idly by as someone drowns, or dies of pneumonia, or infection from a simple scratch. I'm sure we all agree it is wrong to do so. Address the most important problem first.
Like I said, there are some trends that I think are working in our favor. The biggest of these is the strong negative correlation between education, most especially of women, and birth rate. The United States actually has a birth rate below the replacement rate, and would be declining in population were it not for immigration. Just a few days ago there was an interesting RadioWest interview about this. Our birth rate is around 1.9 births per woman, which is below the (current) replacement rate of around 2.1 births. Other countries are much lower, including Japan, as I'm sure you're aware, which is around 1.4.
So this has led social scientists to predict that the world population will peak at about 9 billion people around 2050, after which it will begin to decline. So we've already begun addressing the problem naturally.
Second, I don't think we're near the carrying capacity of the Earth. We have become much more productive and efficient in farming, energy usage (in spite of using more), resource usage, etc. I don't think we're near limits on any of these, though gains will grow harder to get to. But we can hold many more people than we currently do.
I strongly believe we will colonize other worlds throughout space. I think this will be very challenging, though, due to the vast distances and timescales involved, and the amount of energy required, and there are some disincentives to being the person leaving the Earth. Still, there are important reasons to become multi-planet, the most important of which is to make the human species more resilient to natural dangers. Besides, who wouldn't want to be the first missionaries to preach the gospel to other worlds. :-)
I am worried about economic disparity, especially considering descriptions of the Nephites at their best, or the city of Enoch, or the United Order. There is a pattern that has held pretty well, though, that at first new technologies work poorly, are expensive, and few have access, then they become mainstream, and eventually it becomes reliable and everybody has access to it cheaply. Think cell phones for a good example. I can see rich people monopolizing access to important technologies. Yet healthy, biologically young people are more productive and so make better workers, and contribute more to GDP, they get sick less, they are less of a burden on others such as children, they can buy more stuff. I wonder if it would even make sense to give free access to aging medicine for the economic benefits that would result from a productive, purchasing society.