Explanations you cannot put on a slider
Christianity drained civic energy (Gibbon, 1776). Few historians accept it: the eastern half was more thoroughly Christian and survived a thousand years longer.
Lead poisoning, soil exhaustion, malaria. Each is real and each is chronic — which is the problem. They were just as true in 200 CE, when the empire was fine.
Plague. The Antonine Plague of the 160s and the Plague of Cyprian in the 250s hit populations already strained, and demography is slow. Contributing, not sufficient.
476 itself. A convenient marker, not an event. Odoacer ruled Italy as a nominal subordinate of the emperor in Constantinople, the Senate kept meeting, and Justinian would reconquer much of Italy sixty years later.