I was extremely confused about this. When one talks about the Roman Empire, one is referring to the Holy Roman Empire which lasted for 10 centuries and came to an end about the time of the Napoleonic Wars. The Western Roman Empire which came to an end in 486 did not last 1000 years. It started in 286 and came to an end in 486. And nobody I have ever met (and I studied the Roman Empire during school days in history classes) ever thought that the Roman Empire came to an end in 486.

The general resaon given was it went into decline because it forgot to guard its borders and tribes started making incursions, and it just generally faded.