Too bad Drusus’ work in Germany (and that of his brother Tiberius) was undone by Varus.
I don’t buy that the Romans could not have reconquered Germany, as they recrossed the Rhine many times after the Teutoburg, and Drusus’ son Germanicus had some success in his campaigns, but it was probably not worth the cost in resources and lives.
Despite the modern myths about it, Teutoburg was a temporary setback — Rome did reconquer the territory lost, and then some. The additional manpower in the form of auxiliaries was used in the Praetorian Guard (which recruited tall men) and throughout the legions on the frontiers. The Romans maintained their German frontier with four legions, total, and their Danubian frontiers with five legions. It’s astounding, really. They had an active bribery policy to reach the tribes just over their borders, and through trade, travel, and native alliances, to keep the tribes peaceful well into the interior where they never went. Of late Roman colonization has been discerned in the Baltic, as well as in Ireland, and that’s probably just scratching the surface.