What makes you believe that Earth cannot support one billion people, given another 1000 years or so of technological development at our current pace? Assuming

What makes you believe that Earth cannot support one billion people, given another 1000 years or so of technological development at our current pace? Assuming of course: - We've "solved" fusion, and can produce near-unlimited amounts of electricity - Per person energy/resource consumption is cut to an absolute minimum. Most tools, machines etc are designed to last virtually forever, instead of having planned obsolescence - We build farming buildings that are 100 stories tall, and produce light from electricity instead of relying on sunlight to grow plants. - Recycling processes and other activities that are labor intensive today can be made 1-3 orders of magnitude more efficient by AI and better access to cheap energy. - We are able to handle the sociological aspects of such population density Earth's population density is currently at 16/km^2, or 62500m^2/person. If we multiply the population by 100, that's still 625m^2/person. We clearly have enough space, and probably enough atoms (of each type) to support one billion. So I think if we have enough energy, technology and organize it perfectly, it should be technically possible. In fact, given the fact that each American already spends almost 100x more energy than most people did 500 years ago, it may be even easier than is assumed above, if we assume that resource consumption is cut to near subsistence levels. Obviously, such a world would be quite dystopian, making the USSR seem positively cheerful. But impossible?