Maybe this is possible too:

If the hashrate comes down so extremely the 'mining part of the world' would try to get any mining machine as the profitability would go up even for non-profitable miners. They would buy/ship all miners to these places. Even GPU mining would become profitable again (for a short time). So my expectation is that the hashrate would rise again but we wouldn't reach the former hashrates. I can't imagine an average blocktime more than 30 minutes for a long period. The mining rentability would correct it.