That's not really accurate. There are many stakeholders in bitcoin that are part of the ecosystem and all can determine its future. Mining is one part of the equation, there are nodes that validate transactions, there are exchanges that give bitcoins value by providing on-ramps to other currencies, there are merchants who accept bitcoin for goods/services, there are core developers, and finally the end users, investors and hodlers. If miners decide to damage the network through collusion, then it's unlikely the other actors will allow this for long without altering the code to mitigate the threat. What good are these ill-gotten coins if exchanges will not give them dollars for it, or users have no demand for them? Contingency plans have already been developed for this scenario and, so far, they've not been needed because the miners are as invested in the long term success in bitcoin as anyone else.