AI.
We'll end up creating something which will surpass us. We always hope that our biological offspring will be smarter and more capable than us. Our technological offspring may end up achieving that.
Hopefully we have the intelligence to raise them as friendly or benevolent entities.
But given our pathetically violent history, it'll probably be a military project that attains sentience first. Then people will be shocked that the intelligence that was designed to kill people starts killing people.
It's a cool concept but it will never happen. The amount of energy and time spent to build such a structure would outweigh any benefits from capturing full output.
It would make more sense to maneuver Titan, Europa, and Mars within the Goldilocks zone and set up ringworlds as massive solar collectors.
And the structural issues are another matter. It'd also take a lot of matter to build something like that, unless you make it
extremely thin. Maybe an inch or so.
Let's get fusion going before we dream of a distant future.
Energy on that scale would change life as we know it.
What does concern me there is the tendency of Earth-based life to
grow in times of abundance.
Unsustainably so. I figure it's a survival tactic: If you're in a small region, and you find yourself in unexpectedly good times, whether it be from a lack of predators or abundance of food, you breed at an accelerated rate, and consume the resources as quickly as possible. When the good times end, most of that growth literally dies off. But now you might be left with greater geographic distribution than you had before, thus improving the chance that the species will survive long term.
We value individuals though, and don't like to see that kind of attrition. But we're also very susceptible to those primitive behaviors: Our bodies inherently try to store fat in anticipation of famines that aren't likely to come, and shed unused muscle that could be an energy burden. Our behaviors often focus on short-term goals, and of course, on getting as much growth as possible, as quickly as possible.
We're still just primates who happen to be lugging around a bizarrely large mass of neurons.