[Ireland] did not offer potential taxes or anything else worth the effort... Actually, the same was probably true of Germania.I'm sure the conquest of Britain showed how much work would be involved. Agricola's campaign conquered what is now the Scottish lowlands, but didn't offer much in the way of potential, particularly for growing grapes (a major industry). And the German frontier was restive for centuries, making it difficult and dangerous (particularly after Augustus reduced the size of the regular army by half -- not counting the Praetorian) to draw down troops and shift legions for a major campaign.
I do recall reading someplace of some Roman commander actually doing the math of what conquest of Scotland would have taken, and doing the math of what that part already taken had yielded in terms of treasure, taxes, and slaves...and concluded that building roads and forts and chasing down every last naked blue man into the hills of Scotland was never going to pay for itself no matter how long the Romans stayed, because effective conquest would mean depopulation (and deserts pay no taxes), and no Roman in his right mind would have ever gone and settled there.
I think that the Antonine Wall is actually built at the narrowest neck of the island, which made the most sense militarily, but it made no strategic sense because behind it were lots of restless Pict savages who had to be constantly policed, and who didn't render a damned thing to Caesar because they had nothing but hair and haggis.
Hadrian's Wall is built across a wider neck, but has the virtue of having settled Britannia behind it, without walling the "Apache" in on the INSIDE of your fort-line.
I don't really think that conquering and civilizing Germania would have ultimately done the trick anyway. The place would have been like Dacia: exposed, taxed more heavily than its meager economy could bear, and with even more howling barbarians further on. The Germans, at least the Ostrogoths and Visigoths, were mostly Christianized before they broke loose within the Empire. Imagine the border pushed to the Elbe, or even the Oder. What difference? So the Vikings sack Rome instead of the Ostrogoths. You end up in the same place. Maybe.
More likely, the Scyths and Norse raid Germania and tear it to shreds, the Romans eventually retreat, and Germania just ends up being a bigger Illyria.
But the Western Empire might have lasted longer.
Actually, I should have mentioned something else, which may be closer to the REAL reason only a bare handful of Caesars ever could have risked a Germanian, Pictish or Hibernian campaign.
Rome's legions would have won, but to take any of those places would have required concentrating a truly massive army. 8 to 10 legions to completely conquer Pictum or Hibernia. Perhaps 15 to conquer Germania. And once the intial conquest was complete, unlike in the East, large concentrations of legions would have had to STAY there for a LONG time.
Now, here's the problem. It's not economic. It's that any large concentration of a large percentage of the Roman Army in any one place for any period of time offered a temptation...to the commander of that Army. Some Caesars didn't have to worry about this (Augustus, Vespasian and Titus, Trajan, Hadrian, Marcus Aurelius, Justinian, Constantine...perhaps Tiberias or even Caligula, had he not been mad; perhaps Septimius Severus (or then again, perhaps not)). But all the rest did. Having 15 legions concentrated in communications' distance in Germania would not be HEALTHY to most of the emperors of Rome, unless the stronger of them went and took command in the field. But that would have meant practically SETTLING in Germany, since the huge legionary presence would have had to go on, and on, and on. What Imperator in his right mind would trade the Flavian palaces for Frankfort. Even TODAY? Really, 15 legions bivouaced in Germany probably would have been a greater menace to most of the Roman Emperors than the Germans themselves.