If you compare every "forward HQ" from each faction, the main point of comparison is that they are all "weak spots" beeing fragile (ambu/HT/UKFbase) or hard to replace (OKW truck). I mean that they cannot defends by themselfs. Having a mobile reinforcement point, capable of shredding/pinning squads seems wrong on the paper. The nerf was deserved.
I dunno, I feel like if someone wants to spend 120 munitions or whatever the fuck it is now (140? I forget) for flamers or AA halftrack, I think it's fair that it can defend itself.
Considering that the only reinforcement structure in existence before new factions/doctrines etc was the reinforcement bunker (which I never see anyone build, if I'm being honest) I don't really get this argument.
Faction design also matters. Brit reinforcement point is overly fragile (due to brit emplacements being overly fragile to small arms) but it also does a lot more than just reinforce. It has weapon racks, buffs nearby emplacements so you don't need to garrison them and can be used as a retreat point. Ambulance can't be too strong because of how strong riflemen are, so it has a lot of restrictions on its mobile reinforcement. (Must be stationary, in friendly territory.) However, it can also HEAL alongside reinforce which is a huge benefit over the halftracks.
I just don't think it was overpowered. Halftracks were and still are glass cannons. Now you have to make this weird decision between "Do I want reinforcement or do I want a big rolling gun?" and I don't like it. Too expensive of an upgrade to remove unit functionality that the unit is built around.