I'll give the annoying answer that there is no one turn around point. I am one of the believers that it was not one or two decisions from continuing but a deep systemic issue.
The instability of the third century seems to have began the slow process of decentralization, the beginning of fortified villas rather then strong borders. As the borders became more and more porous in the 5th century this exacerbated the problem. This coincided with a general economic decline, which may be a cause or a result of the decentralization. Note that this decline may not have existed to the everyday person in Gaul or elsewhere who may have had the same standard of living, but now relied on bartering rather than the extremly debased currency. As people came to rely on the local strongman in his fortified villa/manor, who offered protection in exchange for service (likely in kind, a share of the crop etc.) for protection it began to become a form of proto-feudilsm and the Western Roman state just wasn't able to transition in a feudal state, as the East would (kind of).
Even if a reunification of the West happened, it would quickly fragment, as it was strong men, not the idea of a "state", that commanded loyalty now. This can be seen with what happened after Charlemagne.
So I suppose we could hypothesize that had the 3rd century been politically stable, the West could have endured like the East, but who knows. The 4th century was one of seeming recovery, but it all fell apart relatively quickly in the 5th.
Edit: The Imperator ninja'd me
