The only savings grace with these gravity systems is that they could last 100 years. Gravity is a very poor of storing energy. Height is limited by practicality. Reusing old mine shafts is just a code word for unaffordable. You need to increase the amount of mass that you move around, therefore your system should be infinitely scalable. By limiting yourself to mineshafts you have reduced the scalability option and the entire project becomes a scam.