Protestors Storm Capital

[Spongebob]

And your analysis is just plain wrong.  Germanic tribes didn't enter into former Roman territories only after Roman power had collapsed in those areas.  Look at Roman Britain,  that wasn't lost until the then governor pulled out the legions as he thought he cold become emperor by force.  And Gaul was largely lost by his actions too because the civil war he started drained the Rhenish legions of their manpower.

It wasn't outside forces causing the collapse of the empire, it was outside forces exploiting the collaps of the empire.

No, you're way off on the timeline.  First off, Rome's military conflict with the Germanic people goes way, way back to the Republic era, but they were mostly able to keep them at bay until the 2nd century CE.  Rome went through various periods of unrest because of battles for power within, but was able to overcome this when a powerful emperor emerged.  The Germanic invasions would be repeated many times sporadically all through this time.  There's a whole other history of the interactions between these two cultures that is complex in itself.  But generally, due to constant expensive wars along the western and northern boarders, Rome's method of dealing with this was to "invite" (meaning do this or else) Germanic people into the realm.  They were initially forced to give up their arms and disperse into different regions so they wouldn't be a threat, regions that had been decimated by war, famine and plague, so they actually needed people to grow crops and they needed warriors.  Eventually some generals became lax on this and the tribes just relocated in Roman territory but kept their cultural ties.  At some point Rome's military became more German than Roman and even the emperors where of Germanic descent.

By the crisis of the 3rd century this has become a real problem, along with economic and currency issues.  But anyway, due to a lot of political savvy and a ton of emperor savagery, the empire lived on another 200 years or so but these same Germanic incursions just continued and Rome slowly but steadily lost it's cultural base in the West.  There were a rotating cast of emperors, some inept but some powerful, but most of them quite savage and brutal.  Of course there were many things going on in the east that were also problematic.  As I said before, it's a very complicated web of causes.

Now the issue of Britannia is a little different and you are right but just not delving into the details.  In fact, Britannia was "lost" and "regained" several times as the Roman governors sought to utilize their considerable power to rise to Emperor, all while Germanic forces and northern tribes invaded the province repeatedly.  It's an entire history in itself.  The problem was that any governor who had enough manpower to hold onto Britannia also had enough power to challenge Rome itself, so this scenario repeated itself several times.  But Rome didn't "give up" on Britannia until 406 CE when it just didn't have the firepower to hold on to all of its distant provinces any longer.  By that time the western empire was just about in freefall.

But in general, you can't say with real certainty that any one thing alone ultimately caused the collapse of the western empire.  It was a combination many things over a vast amount of time.  With an empire so vast and so diverse and challenged in so many different ways, there simply is no one reason for it's failure.  In reality, Rome faced dozens of challenges over it's lifespan and solved many of them quite well, but ultimately they added up and just overwhelmed the empire.  Even so, one could argue that the "idea" of the Roman empire continued to exist for centuries after.  The eastern empire thrived for centuries and empires in Gaul (France) and Germany all claimed Roman heritage for many centuries after 476.

I'll let it rest there, as this thread isn't about Roman history, which I have an insatiable interest for.  You can start a new thread for this if you would like to continue the discussion.  I never claim to know all there is to know about Roman history and I'm always happy to learn more.