There are tons of similarities between the two empires, so perhaps assimilation might not have been out of the question. Its possible that Justinian, rather than pulling Belisarius out of the east after proving himself, he builds up a massive army and has him keep going. He might not be able to absorb everything, but it would be enough to break them
There's zero precedent for "building up a massive army" in our period, though. East Roman armies in the sixth century consisted of between five and twenty thousand men. The figure may have been a bit higher pre-plague, but I think a single army of more than 30,000 men is pretty unlikely to have ever really existed. And 30,000 men ain't gonna be able to swallow Iran whole, especially under such a ruler as Khusro I.
Swallowing Mesopotamia brings a tonne of problems for the Romans too. It's got large Nestorian and Jewish communities who'll resent the Empire. If Chalcedonians and Monophysites could just about be balanced with each other, Nestorians were certainly considered to be beyond the pale, and imperial legislation suggests as such.
And finally, logistics, logistics, logistics. Late Rome, like early Rome, was essentially a state based around the Mediterranean, which means campaigning in Mesopotamia and Iran is always going to be a logistical stretch.