Anyone who runs a full node can decide any set of rules they want to follow. You can incentivize miners to mine on your fork by providing block rewards. If you pay people to do something, at least some of them will.

The ETC fork would have been successful at any non-zero support level from people with the ability to mine. And since any CPU can mine (just inefficiently), any fork can get a mining baseline. That may not give you a ton of security but it works.

People who run full nodes are the ones that decide hard forks.