It doesn't take a genius (well, actually a little algebra would help) to realize that growth in perpetuity (the essence of a global economy, represented in the long run by a line with a positive slope) is not sustainable for a planet with fixed resources (land, oil, etc.; represented by a flat line).
In theory, even if we found a second Earth, we would quickly overpopulate that planet because of how exponential growth works. A second earth would go from 0% to 100% capacity in whatever Earth's doubling time (70 divided by % growth) was at the point it reached 100% capacity.
I'm not going to pretend to know how this will play out, but I'm guessing it won't be a lot of fun. The good news is most of us will probably be old or dead by the time the global economy collapses since our institutions continue to prop it up and push the day of reckoning further down the road.
https://www.youtube.com/watch?v=O133ppiVnWY
PS def don't watch this unless you enjoy crippling reality