I think the main problem of the West empire surviving is that it was simply a growing mess that would get worse before it got better, the costs of having to safeguard such massive borders when their return where so little was both steadily devaluing the coinage as well as making the military much more of a political institution, something that went hand in hand with a growing feeling of regionalism by many areas as they felt that the fat and lazy emperors in Rome were not doing their proper job compared t their local, dynamic generals who defeated the ever raiding Germanics and was a known face in the province... This guy sounds like a better emperor material if you ask me, especially compared to the Neros we had...
Thus, Third Century Crisis.
Afterwards, the West was on borrowed time as the weakness of the empire were made clear and even the reforms of Diocletian failed to prevent the slow death of the Western empire as the constant damage thanks to several raids, the still huge cost of defending such a massive empire and other factors(plagues that ravished the empire, bad harvests, wars with the Persians, Christianity) eventually made the West simply unable to be kept as it was too much cost, too much land to defend and too far away while it gave so little in return.
Compare that to East where it was much more wealthy, much more easy to defend(despite the border with the Persians, they usually agreed to the peace treaties and are easier to deal with diplomatically compared to the Germanics) and was more compact and easier to communicate and even then, it had it's own issues that eventually became really apparent in the Byzantine era, even without the rise of Christianity in the empire, there was a real sense of alienation between the Greeks who ran the empire in Constantinople\Alexandria\Antioch compared to the locals of Syria, Egypt and Palestine who resented such dominance and thus were easy for the Arabs and Persians to help during their conquests of the Empire and Byzantine never managed to regain those areas back in the case of the Arabic invaders, showing that even the mighty east had institutional problems of their own.