The Crisis of the Third Century, which starts in 235, is where the inflection point between "broadly stable" and "broadly negative" sets in, and the shocks both of the Crisis of the Third Century and the Plague of Justinian are each larger than the shock of the deposing of the last Western Roman Emperor.
However, the point I was trying to make is that Rome's decline sure lasted a very long time indeed.
Additionally, 235 being so important is only obvious in hind-sight. To anyone living in the Empire in 235, I suspect it felt an awful lot like the Year of the Five Emperors (about as far away from 235 as Diocletian is the other direction) and it wasn't until a while later that it became clear that no Septimus Severus type was going to be able to put it all back together quickly.