Historical consensus favors the plague as quite significant, even if the city of constantinople managed to survive independently for several more centuries. The weakened mediterranian presented opportunities for the gothic tribes to take territory in gaul and italy in the decades following, and the economy nor the manpower of the empire never recovered. By the fall, Constantinople was a hollow shell of what it was, controlling hardly any territory and partially in ruins, ultimately abandoned by its few remaining allies in the face of the Turks.
You are way way way oversimplifying nearly 1000 years of history between the plague of Justinian and the fall of Constantinople.
The Justinian expansion was untenable. If you look at it on the map - there are strong enemies on literally all sides. It was a desperate but hopeless attempt to regain the Western Empire.
The plague made it worse - but was hardly the catalyst. ...and the East Roman Empire was far more than a city state for a majority of the remaining NINE centuries.
