The thing is while both the the East and West suffered from similar problems, the East had the resources to create a margin of error, where the WRE didn't.
In order to have the West survive, you'd have to have WRE make slam-dunk after slam-dunk until either an Emperor solves the West's problems or the external threats die down. This is very difficult and neigh improbable, but not impossible.
As for the East invading the West: When Julius Nepos died in 481 the title of Western Emperor defaulted to the Eastern Emperor and, until Justanian came to the throne, the Emperors in Constantinople were content to let barbarian kings rule Italy, theoretically in the name of the Empire.
Without the fall of the Western Empire Constantinople doesn't have that excuse and plenty of excuses to focus on the east with the rise of the Sassanian Empire.