You conflate rather different cases here. Scythia is hugely more remote and vast than Germania or Mesopotamia, and I would never dare argue that Classical Rome may conquer and hold it. As it concerns Germania, there was no such thing as constant pressure of Central Asian nomads on it. That kind of breakthough only occurred once every few centuries at the very most. To lose Mesopotamia weakens Persia considerably and nets Rome a much more defensible border.
As I understand the term Scythia, it can be used in a variety of contexts. I'm mainly talking about your contention that Rome can expand up to the Vistula-Dniester; the eastern part of this area beyond the Carpathians I would call Scythia. Am I mistaken? In any case, the entire belt of territory from Frisia across the North European Plain and southeast to the Black Sea is hugely remote from the center of Roman power in the Mediterranean. In fact, "Scythia" would probably be easier to exert control over than Germania. To say this is a more defensible border is taking into account just the geographical length. The time and effort required to transport troops and settlers this far away from Rome compounds the problem. The regions that Rome expanded into OTL were already at least moderately populated. These new regions are almost totally empty at this stage, and the Roman demographic base is not bottomless. The best Rome can expect to achieve across the board is probably a loose network of trading and military outposts such as existed in northern Britain OTL. Certainly Romanization will have hardly started by the time it must inevitably close down due to internal decay of the Empire.
The civil wars that steadily plagued the Roman state are going to put additional immense pressure on Rome to abandon these largely useless regions. It's also a given that the large number of legions in these provinces will be incredibly prone to revolt due to their distance from Rome. It's basically a massive reservoir of troops for any general with the compunction to march on Rome and take power away from a weak Emperor.
To hold Germania mostly empties the demographic pool of unassimilated Barbarians, apart from the nomad breakthroughs that only came twice a millennium or so, and net Rome a much more defensible border. The notion of remoteness and sheer size of Mesopotamia is laughable, and Germania is similar in size to areas Rome successfully conquered and assimilated.
There will always be pools of barbarians on Rome's borders. If anything, a more successful Rome means more developed barbarians as all that economic prosperity will spill over into unconquered territory. This happened OTL. In the days of Augustus the tribes in Germania were incredibly primitive and thin on the ground, but within a few centuries they had fed off contact with the Empire to become an incredibly threatening enemy. The same will happen to any barbarian group on the borders of a thriving civilization. How do you think the Romans got where they were?
There is no overextension when you move from a less defensible position to a more defensible one that is within the same general area.
Defensible from external enemies, perhaps, but what you fail to realize is that the bigger the empire the less "defensible" it is internally from the internal stresses which are the real killers. A shorter border isn't going to save Rome when rogue generals are taking it apart piece by piece at its core. It'll help even less when the currency of the financially exhausted Empire is totally devalued, and the economy broken and decayed to OTL Dark Age levels.
Invasion at home ? Excuse me, apart from the Germanic tribes and Persia, and the steppe nomads that came once in 400 years, who's going to invade Rome ? Native Americans ? By the time the Norse are scheduled to come, Germania and Mesopotamia shall be as Roman as Gallia or Anatolia.
Apart from the northern barbarian tribes, Persia, and the inevitable waves of nomads, no one. But to think that Rome can eliminate any of these threats by these extra conquests is misguided. For the reasons I have stated above, Rome will decay all the faster for having so much dead weight on its shoulders sapping its military and economic resources. As decay sets in, no amount of uber-defensible border fortifications are going to prevent its enemies from breaking the dam and storming through.
When the Qin Dynasty first expanded into Southern China (e.g. Guangdong, Fujian, Hunan, etc), the existing population was not "Chinese". They were Baiyue who spoke Tai languages. While they never threatened the Imperial rule itself they were able to make governing the area challenging. It took well until the Tang Dynasty almost a thousand years later to fully assimilate the area, and even to this day traces of the Baiyue remain in the language and culture of southern China (e.g. spoken words in Cantonese which cannot be expressed using proper Chinese writing).
So it certainly is possible for the Roman Civilization to gradually "Romanify" Germania and Mesopotamia over centuries, though it's probable one of those regions will pull a Vietnam and break off, develop its identity distinct from the Romans before it had been fully assimilated. Given that Germania is much more flat and accessible than the Lingnan region of China it's probable that it will assimilate faster than its Chinese cousin. Scythia, however, is too far for any empire with its core on the Mediterranean.
Even Rome OTL managed to Romanize large parts of Western Europe that were within its logical historical range. With a stronger demographic base, and a more stable strategic position China was able to go quite far. From the Yellow River valley to all of what we consider the Han region today is quite a leap, but China was essentially pre-destined to expand this far by the regional geography and its advantages over the tribal groups who lived in the assimilated regions.
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