> Second, if most miners stop mining a coin, it can kill it.

This won't happen though. Miners are out to make a profit, full stop. They mine whatever currency is most profitable, so hash power follows price. Bitcoin has over half of the total cryptocurrency market cap. So long as Bitcoin itself is highly valuable, the miners will naturally follow along as well. And as some miners temporarily defect (thus incurring massive hourly losses in opportunity costs from mining less valuable currencies), transaction fees on Bitcoin simply go up higher, making each block worth significantly more reward, thus tempting the defectors with ever-increasing profits to defect back to mining Bitcoin. There is no mining cartel.