Your math is off here. If you bought those 70k S9 units that would be about 910 EH/s. What your forgetting to take into account is that would now push total network hash to over 2300EH/s. (Network currently sitting at 1500EH/s not 1600). So 70k isn't nearly enough. It would be more like 125k S9 units (estimate here not doing exact math).