If no one mines, the difficulty decreases to where you can mine on a laptop again. People will do it then, like they did at the start. A 51% attack is designed

If no one mines, the difficulty decreases to where you can mine on a laptop again. People will do it then, like they did at the start. A 51% attack is designed to be less profitable than just mining it. But maybe some gov does it to try to shut it down, well then, the chain gets forked and we go on with our lives. That's the point of everyone having their own node.