I did not say that statement is untrue for the modern "Ukrainie". I said that the part of modern Ukraine with the good soils has almost nothing to do with the Kievan Rus. Without going into the details, using the term "Ukraine" in the context of X - XII century simply does not make any historic or geographic sense.
"Pontic Steppe" is not Kievan Rus ar all.
To make it short and clear: it was absolutely unrealistic for the Byzantines to conquer and secure the Wild Steppe. Would not and could not happen because this would require combination of the massive well-garrisoned fortified lines (which, as OTL experience demonstrated, would not provide a complete guarantee against the raids), massive resettlement into the territory and ability to maintain and expand this effort for centuries. Fighting people of the steppe was not a problem. The problem was to prevent their raiding and this was not possible until the modern times even with the sedentary neighbor states being much more powerful than Byzantine Empire (and Kievan Rus) and better suited geographically and demographically for accomplishing such a task.
Can you not cut out the parts of my post that explicitly address your concern? It comes across poorly. I finished the post with an explanation of the caveat of needing to develop a model, for which the Pruth is a great place to develop one on a smaller scale. It could be any number of things - small fortified settlements, aggressive pre-emptive warfare that doesn't just raid but effectively kidnaps entire peoples that even come close to the frontier, it could be the "clientelsiation" of nomadic people to build a network of alliances that punish members of their own community that raid. As of yet, I don't know. It could be all three and a great bloody wooden wall along the length of the Pruth.
As to the rest - ok, so Ukraine is an anachronism. I was referring to the lands South of Kiev, and West of the Dneiper. Ukraine was a shorthand for it.
As to the use of the Kievan Rus - I don't care. I didn't introduce the Kievan Rus into this discussion, you did. I have been discussion either Kiev (the City), or the lands south of it.
As to some of the factious postings afterwards -
you are mixing motivation and advantage. The thread was focusing on the results. If you want a motivation (since again, you aren't actually providing constructive critique, a pattern that I'm noting an objection to here) then, ending the raids. It isn't implausible that the Romans decide that it is worth some effort to take a stance to end raids across the Danube, and at the same time use new lands to both pay soldiers and establish new recruitment areas for troops from the descendants of those settled.
Motivation: Raids into Roman territory are costly, and the Danube is a longer border than the Pruth, and guarding Carpathian passes. Thus securing the northern border is the goal.
Action: Campaign into the region, taking any major population centres, and either capturing (and resettling elsewhere) the people who live there who aren't already settled, or killing them. Up to the commander. Continue campaign until the Pruth is reached and the area is pacified. Adjust strategy and tactics to fit - as you've stated, fighting them isn't the problem.
Expected Result: With the Pruth River and Carpathians as natural frontiers, the Romans can fortify the area, and settle soldiers in the region. The lands south are secure, and are less raided, and can provide more taxes. (Also, fewer soldiers needed on the border). Also, shorter border that raiders from the steppe can come from, meaning that if raids are limited by distance from the frontier, less land that will be raided by these peoples.
Unexpected Result: Better than expected harvests that can lead to a local agricultural boom (and whatever minerals are found) - which can more likely lead to supporting both the Pruth Frontier, and elsewhere.
Does this hold true for the Vistula? I don't know. As stated repeatedly. I however don't take the opinion of it being impossible. It may well be that the Pruth strategy doesn't scale, but instead a campaign that goes further (trying to both capture Kiev and mimic the results of a hypothetical Pruth frontier on the Vistula) fails, and instead you develop a large "Cossack" region. Of dubious loyalty to Constantinople and Kiev, but hard to control, and as liable to raid Romans as anyone else. That could lead to a withdrawal, or it could lead to trying to bring the Cossacks on side as a preferable group to the other people who live on the Steppe. At least these "Cossacks" are Christians.
I quite like the idea, but I disagree on its feasibility, mainly around the point of Kiev. Sure, it isn't going to be Greek, I think it'd stay some form of Russian. However, I think that Kiev, it taken, can easily be garrisoned, and then supplied by river, as at least by my research it isn't hard to reach, even in winter, by ship from Constantinople, the only difficulty being that in bad years you may want to have "Icebreaker" galleys in place.