Paratroopers take up 9 population and they can take on obersoldaten on a straight up match, which obersoldaten takes up 10 popcap so I think Paratroopers are at a fine spot.
Do you give your obersoldaten orders to run circles around the airborne? Because that's pretty much the only way to make them lose to airborne.
Vet3 airborne can kinda beat vet0 obersoldaten, they'll only lose ~4 members while the obersoldaten lose 3 and are forced to retreat. Glorious victory of 38MP. Vet2 and onwards the obersoldaten just piss on the airborne though.
As to the topic at hand, I feel that the 9 popcap for airborne is justified by their benefits over riflemen (7 popcap). Their popcap is not something I see as an issue.