Here´s why someone would go for Rangers and Airborne: You don't bleed like a pig and you get quite effective weapon uprades. 120mun for sniper lmgs and 90mun for terminator machinepistols.
Obers and fallschrim bleed like a pig, cost munition for a ammo intensive faction (ober only), and still only has questionable effectiveness for it's "blob control/ anti infantry role". Their LMG/ FG-42/ Kar-98 is not sniper, not against vet 3 rifles/ rangers, they dont get 2 MG-34s either because they have no ammo rack.
Nor are they effective at close range because they are LMG/ Rifle armed, or using a weapon with questionable performance. (Fallschrimjager FG-42....? It feels weird, the damage it makes is highly inconsistent, it either shoots up a squad of conscripts in seconds, or gets overwhelmed by that conscript after missing volleys after volley)
Either Obers and fallschrim get fixed, and does it's role of "elite infantry that hunts other infantry", or volksgrenadiers must pick up the role that Ober used to fill, or at least compensate.
Your post confirms a few thing only, and it is that OKW elite infantry are over priced, under powered, and generally unappealing compared to simply getting 2 more volks/ Pzfus to fill it's place.