Marcus Aurelius was the last of the Five Good Emperors because he did not adopt a competent, non-biological son to take his place like the previous four. Instead, he set up Commodus as Caesar and his heir, despite his mental instability. That decision alone calls into question his Stoic resolve.
True, the other Good Emperors - Nerva, Trajan, Hadrian, Antoninus Pius didn't set up their children as their successors. They each adopted someone who would be good at the job. But there was one difference between them and Marcus Aurelius - none of them had biological sons. Their adopted son would be their only heir.
Marcus Aurelius' decision can be criticised in hindsight because Commodus was terrible at his job. But I'm not sure I could have done differently in Marcus' shoes. Parents find it difficult to view their children objectively and feel the need to protect them. Even if he was aware of Commodus' faults he also knew this - if he adopted someone else and crowned him Emperor, then it would have led to civil war after his own death. Either Commodus and his other sons would kill the adopted son or vice versa. Having all of them alive and at large would be an unstable equilibrium that could only be solved with war.
Come on man, this guy ran an Empire pretty well for a couple of decades despite challenges like war and plague. Maybe he knew what he was doing. Give him the benefit of the doubt.
