Why Rome did not Lasted

Rome collapsed due to internal reasons, then it should be some kind of very different Rome to last this long. Theoretically it could end up like Byzantium and simply fall later.

Thing it Rome could have prospered and be eternal only if it stayed in its Golden Age (Augustus Era). However, changing socio-economic factors led to its decline. Even in 200AD it was a mess, in 476AD Rome just fell on its back die to exhaustion and refused to move.

Even before this date Emperors relied on German mercenaries to do actual fighting, however after that date they could not even find a competent enough local general, so they promoted leader of mercenaries to a de-facto emperor. Of course, new security soon realized that there is no one left to enforce terms of their contract so they can run the place as they see fit.

East Rome (Byzantium) could have interfered, like they did during the Justinian time. However, most thought that relatively poor and unprofitable West just is not worth it compared to much richer East. So, they just let Germans have it.

It's not the first time Western military elite simply dumped their home countries and relocated themselves somewhere more affluent and profitable. Take Diadochi for example. The just ruled in modern Egypt and middle east and could not be bothered to take their home Macedon back from even Greek city states, much less Rome.