I guess what I meant by that was that it ceased innovating from a civic and technological point of view. There’s no space for it. Once you get the other side of the Five Good ones and Commodus, you’re dropped into a period of relative chaos. For a state to innovate requires a period of stability and this was lacking across most of the 200ADs. You get a period of stability around Valerian and Diocletian but they’re never established enough to innovate from and Valerian spends most of his time putting out the fires other people have started — Constantine splitting the whole thing in two around 330ish pretty much seals the fate of the West.

Most innovation comes from citizens well enough off to be somewhere decent on Maslow’s pyramid — that isn’t happening in the West. There’s too much turmoil politically and militarily and not enough focus on civic projects. The cost of being actively involved in politics now outstrips the earning power of the middle class and it becomes an upper class pissing contest about who has the biggest army. Throw in the fact the Eastern half of the empire isn’t all that bothered with Rome as anything other than a symbol and by 476AD you’re done. The Turks get ‘Turkish Baths’ and the Western end of the world gets ‘the dark ages’ where people hold bits of Roman technology and look puzzled.

Innovation is a product of stability. Stability is a product of continuing to safely implement innovation. We’ve all had to be a bit more careful these days since we’ve got the capacity to end the world now. Wouldn’t want to have given Caracalla a tactical nuke ;o)