Probably true, but it was so baked into the system that it was hard to escape. Even the five good emperors chose their successors via adoption, so on paper it was still direct inheritance. For Marcus Aurelius to pass over his son would have been a huge break from tradition.
The best attempt to escape this was Diocletian's tetrarchy, but that just led to yet another civil war.
The best attempt to escape this was Diocletian's tetrarchy, but that just led to yet another civil war.