Here's my take on things:

(1) The Rome we are talking about was never a fixed thing.
(2) Rather, Rome was continually reinvented as a different thing (if you look at it closely enough) with the same name.
(3) 'Rome', in this sense, ends whenever we (now, looking back) deem the new invention of it to be TOO different to justifiably call it 'Rome'.
(4) The doom of this 'Rome' is whatever proximal event precipitated the new 'non-Rome' reinvention (whether this is Augustan reforms, too many barbarians, Attila or whatever).
(5) Rome, as the notion that it is a cohesive thing, is doomed to change, from inception.
(6) You will never get anything other than a mish-mash of pretty much everything can be plausibly linked to have doomed 'Rome'.
(7) But that is not what you want to know.
(8) What you want to know are specific things like:
What stopped making it possible for groups of men (wearing red and Lorica ______ta) to be organised and fight other people for the defense of boundaries on a map? Or,
What stopped making it possible for certain acts to be procedural sanctioned through a codified set of laws in southern France? Or,
When was it no longer possible for caartographers to draw maps that included a huge red chunk covered with "ROME"? Or, whaterver.
(9) Each of these questions will have complex answers, but answers they will have.
(10) When was Rome doomed depends on how Rome is defined, and the more complex this definition (eg. all of the above) the more complex the answer.
(11) Hence there are many wonderful stories to answer 'When was Rome doomed'
(12) And life is about stories after all, and this is a good thing.
(13) Thanks for the question.