if there was some secret cartel of "lets make md5 blocks for the next 6 months to force nodes to be less secured"
initially 20 pools have that motive.. but soon enough a pool gets greedy and jumps back to sha256 and wins every block.
Your example of MINERS changing the hash algorithm is of course somewhat strange, because they would kill their own hardware. But if you want to prove the power of full nodes, you will have to leave aside hard forking. Hard forking will be decided in the market ; NOT by full nodes. So this is a straw man argument.
Try rather to argue how full nodes would impose MD5 mining upon miners who continue to use SHA-256. THEN you will prove the power of full nodes over miners !