However, market forces guarantee that the difficulty will go up to the point where it's infeasible to add more hashrate, e.g. because it becomes uneconomical or there's simply no more hardware available. PoW workchains must work at maximum inefficiency. (This bears out in the data, too, I think. BTC had its highest hashrate at the beginning of this month, when energy price were at an all-time high and BTC was at the lowest over 12 months)