That would be certainly be true of the Western half of the Roman Empire. The Eastern Roman Empire fared even better and held on until 1452, when the Mohammedan Turks, with the assistance of Italian artillery designers, finally breeched the walls of Constantinople that had stood for 1000 years. (Built by Theodosius II).
Diocletian set up the tetrarchy of course. He also carried out one of the last - and most brutal - persecutions of Christians in the Empire. His fellow tetrarch Constantine ended up issuing the edict recognizing Christianity as a legal religion later on.
Like I said, the guy did make some mistakes - a couple of whoppers, too. On the other hand, it's very hard to argue with his results as a whole.