All other factions with all their other AA have alternatives or suplimentary AA.
USF has pintles.
OST has pintles and the 222
Ukf could build a bofors
OKW has free AA on their base, can build multiple 2cm pits and has pintles
Soviet can have 1 pintles on field max, and they are attached to the most expensive tanks in their entire roster AND are doctrinal, limited to 1.
ALL other AA is also more durable or survivable.
USF can disembark and screw up tracking (can also use this to generate vet for other tanks or give themselves a Vetted AAHT)
UKF centaur can bounce shots and has a large health pool making it impractical to dive
Ostwind can bounce shots, has health and has mobility
OKW has smoke and again that free AA on their base which can't be stressed enough as bonus AA.
Correct, but other factions need significantly more resources to achieve the same AA result. Also for teamgames allies have stock recon in USF major and UKF officer, and much more artillery options to gain accuracy from sight, so it's much more binding for axis to invest in AA. OKW base AA is not so useful on most teamgame maps since most recon will pass from allies' territory first.
For the halftrack arguement, both halftrack AA variants (assuming OKW and USF ones) have an early timing where AT threats are not present, so they can do a couple pushes unthreatened. That doesn't mean they are used as shock vehicles, they might give some direct anti inf advantage early on, but they always end up guarding flanks and shooting planes exactly as the quad. They are also way more expensive, and can't be used as reinforce platforms.
M5's role is also reinforcing in the mid/late game, similarly how you would get a 251 as wehr so it never gets obsolete (esp with the proposed med crate changes). If you want AA for the late game, you should invest similarly to other armies in price and pop.