they are submerged in a murky complacency loop of lack of success

Well, not finding success is not necessarily because of self sabotage.

If people are mentally blocked its not easy to make them change their perspective of things. Perhaps saying things directly (about lacking of success) in a semi harsh way will make them wake up and start changing their mentality. But its dangerous because if you speak to harsh on them this can negatively affect the relation and even make the other person feel bad about all this.

But being honest when talking about delicate things is probably the best in my opinion, being honest and direct.