Both you and the person you're replying to are correct. These are not mutually exclusive factors. (Well, except the part about egypt: that was assimilated after Anthony and Cleopatra's defeat, not after Pompey's defeat. Caesar allowed Cleopatra to remain a client monarch.)
Interestingly, that disasterous campaign you mentioned by Augustus to try to conquer up to the Rhine was done exactly to address the difficulty of defending Rome's extended borders. He felt that it was important for the empire to achieve its natural borders, just the same as France sought to do after the revolution.
However, even had those natural borders been secured, it would have done nothing to address the internecine strife that was was instrumental to the decline and final collapse of the western empire. It is necessary to look at a combination of both internal and external pressures to explain what happened.