Rome stays a great city on par with Milan
Well Mediolanum in otl was brutally sacked by the Goths while Rome was spared. Rome as an urban polity collapsed later beginning with the reign of Justin II where the Lombards invaded Italy. Though its not really clear if they invaded as part of a desire for conquest, or a botched attempt by the Roman government to try and reach a compromise with them and settle them in Italy.
Rome during the reign of Constans II was not treated very well. Constans II was intially very warmly welcomed as the Emperor personally visiting was a huge deal, but instead of lavishing the city with honors, or contributing much to it, he basically stripped it of anything valuable as part of allocating finances to fix the Empire's fiscal crisis (thanks to war with the Arabs) and his ongoing campaign against the Lombard Dukes in Southern Italy,
If it doesn’t happen because of a Roman noble then it’ll happen when one of the either two Germanic monarchies are sick of being a Rex and want to one up their Germanic neighbors. That is unless Theodoric’s descendants don’t conquers them in true in Roman form.
Well that depends if the Romans reconquer region or not. Plus even if Theoderic was able to unite the Gothic nations under his suzerainty, he still was scared of the East. While it was busy dealing with threats on the Danube and the Eastern frontier, when it got its act together it was a force to be feared.
After all just look at the reign of Emperor Leo. He put together one of the largest naval operations in antiquity and would have almost certainly annihilated the Vandal Kingdom had it not been for the sheer idiocy and incompetence of Basilliscus.
And even if these Kingdoms united and got too large for Constantinople to safely crush with military force, they have other means. The Romans could literally throw money and arms at their enemies which was one of their favorite tactics (the Chinese used a similar tactics against the various steppe tribes to prevent them from unifying). They did this when Odoacer got too high and mighty prompting the Emperor to send Theoderic to crush him. Emperor Manuel did this against Frederick Barbarossa who was consolidating Hohenstaufen Rule in Italy and successfully bankrolled the Lombard League with funds and arms. The later Emperor Michael VIII Palaiologos did something similar when he orchestrated the war of Sicilian Vespers taking Charles of Anjou down a peg leaving the Kingdom of Sicily permanently fractured into two bitter rivals.