Let's say near the end of the western roman empire, the emperors decides to pull back all their frontier troops and etc, abandoning France, spain and etc, and focus their effort in defending Italy alone.
Well, that's pretty much what
did happen. Not necessarily by choice, mind you, but by the time that Odoacer deposed Romulus Augustus, the empire had already been completely expelled from Spain and most of Gaul. The only non-Italian territories under nominal imperial control were smallish military outposts in northwestern Gaul and on the western coast of North Africa. Neither of these were contiguous with Italy, and imperial "control" was little more than a formality in both cases.
Will the western roman empire be able to defend Italy alone? A Italy where the pope has less influence than the western roman emperor.
Probably not, no. One of the many issues was the Roman army really depended on barbarian troops. The people who'll be actually
defending the peninsula, in this timeline as well as ours, are going to be barbarians in the first place. And, if nothing else, an Italy-only empire is going to have a hard time commanding the fealty of a better armed, and probably wealthier group of settlers.
You might (
might!) be able to have the eastern empire hold onto Italy for a longer period of time following Justinian's reconquest the following century. That is,
if the fighting doesn't completely devastate Italy, and
if there's no plague and political infighting to distract Justinian, as there was in OTL. But the results of that are going to be considerably different than if the western imperial infrastructure never fell in the first place.