How exactly should society enforce laws with no police? Mob Justice?

We seek to replace things that are not working all of the time. Sometimes it takes more creative solutions, but usually we can fix systemic issues we have created ourselves, as long as we have a little imagination.

Defunding the police would allow us to do things like fund more social workers and non-armed, non-enforcement officers who can help people in crisis. It would allow us to fund infrastructure for safer streets, classrooms and highways. It would allow us to fund restorative justice programs, subsidize housing for those who need help and make our society work for everyone. It would allow us to fund drug treatment and education programs, fund school lunches, and help create vocational training.

Fundamentally, defunding the police would allow us to actually identify the problems that they solve, and eliminate those they create. The problems that are left over can then be solved in different ways.

Defund doesn't mean destroy. It means decrease funding.