We can safely write that the decline was multifactorial: weather, economics, external military pressure from other nations, internal social decay, agriculture, and other variables had parts to play.
Did the final collapse of the Empire come as a complete surprise to its inhabitants, or did they see it coming, and brace for the impact? Here again, the evidence is mixed, but it seems that at least some of the citizens realized that the government was about to crumble.
Those with foresight understood that, if a government is soon to vanish, then its property is available to anyone who has the physical means to take it. Building blocks were gleaned from many of the large stone buildings in Rome.
Landowners simply sent their servants to disassemble parts of those structures, bit by bit, and use the material to build other, privately owned, structures. Historian Thomas Cahill writes:
Between the Sack of Rome by Alaric in 410 and the death of the last western emperor in 476, the Imperium became increasingly unstable. The large landowners - more and more, laws unto themselves - ignored the emperor’s decrees, going even so far as to use the great public edifices as quarries for private palaces. Rome itself, abandoned by the emperors for the more defensible marshes of Ravenna, saw the splendor of its public buildings crumble before the destructiveness of private greed. Though the emperor announced dire punishments for any official who cooperated in this destruction - fifty pounds of gold for a magistrate, a flogging and the loss of both hands for a subordinate - the looting continued unabated. The Vandals were not the only vandals.
The authority of the emperors was undermined by several factors, one of which was their absence from the city. If they remained sheltered in Ravenna, and content merely to send orders to the city, then the residents of the city felt ever more free to ignore those missives.
The end of the empire was an at least partially peaceful and orderly transfer of government: the Germanic tribal leader Odoacer was acknowledged as the ruler of Italy by Byzantine emperors Zeno and Julius Nepos, and Odoacer worked collegially with the Senate.
It is possible, in fact, that some saw the end of the empire as an improvement: no longer was any effort wasted in sustaining the facade of imperial authority - an authority which had vanished long before its pretense finally expired.
Odoacer - whose name is subject to various spellings - may have represented, in the minds of some Romans, a new start. His rule was largely successful. Given the increasing numbers of Germanic soldiers working for Rome, Odoacer may have had a better chance at leadership than a Roman.
Theodoric eventually assassinated and replaced Odoacer. The violence of imperial succession outlived even the empire. Theodoric was from a different Germanic tribe than Odoacer, but their successive reigns showed that the Germanic tribes were now in control of Rome, and with it, most of Europe.
