What if people try it instead of a scientifically supported treatment and then it doesn't work and they give up and get addicted to opioids when a real method would've worked for them?

Don't get me wrong I think the placebo effect is a very useful tool but best to combine it with other methods that work for many back conditions, such as yoga routines that strengthen the back muscles. Of course what works depends on your particular condition which is why anyone with severe back pain should see a medical professional to get advice for their specific problem.

Actually there is pretty limited support that placebo actually works on anything but subjective measured outcomes.

Like pain?

Back pain is mostly subjectively measured outcome, and many types of back pain respond very well to placebo.