If china decided to stop mining and segwit activated with payments not going to miners and through payment hubs instead. Mining would not be profitable and very few new blocks would be rewarded. Very few countries have electrical grids that can supply electricity at a price that would be profitable to 1.) start manufacturing asic chips 2.) setup new bitcoin mining operation 3.) Once running, not drown in electrical costs and forget upgrades. I think another coin would have to rise, but existing bitcoins might be able to survive albeit very slow.