the thing is though, it isnt going to be doing anything at all but AA. the 222 retains scout ability and if still armoud probably already earned its keep 10s of minutes ago when it was serving as a LV hunter/shcok unit for its timing. the quad has no such luxury. its the most cost effecient because thats all it is full stop. if there is a quad out it was a move to do that and that alone, all other AA from the 222 to even the emplacements generaly are built for more than just AA aas they bring more than just AA to the table.
you more or less created vehicle crews aside from wanting sweeping too and they exist.... without the additional cost.
every faction has a few boons and areas they are cost effecient in, the only AA the soviet can muster hardly seems an issue being one of those things when its so easily taken out imo. for example that 222 we mentioned from earlier can be used to dive and take out the m5 should the time come but i have yest seen anyone dive an m5, despite the similar costs to take out even something greatly more valuable like rocket arty.
Vehicle crews is quite separate as their cost cannot be detached from the vehicle itself. Dismount the crew and your vehicle is useless until manned again, you have no net gain in unit count. But imagine if you could buy vehicle crews for 120mp and upgrade them with sweepers...
Now regarding the first point about M5 ONLY being able to do AA. I dont care if it's only able to do AA, there is a limit that it shouldn't exceed. If you removed the elefant's MG, it would not justify increasing its damage per shot to 640 even though the only thing it can do is anti tank. The M5 exceeds a potency limit for cost. That it is better at AA then 2 ostwinds which cost 4x the pop and 7x the fuel is a problem even if AA is the "only" thing it can do-which i don't even necessarily agree with in the first place.