The issue shouldn't be that the M16 isn't a 'suitable' replacement for the Ostwind. A 'suitable' replacement means (I guess) one that can kill planes with little or no skill required, kill GVs with little or no skill required, and be the best at killing.. well, anything.
Lets look at a related issue. The F4U-1C was either perked because of overusage (20% of plane kills, I believe the figure was), or rarity in real life (200 produced, and I don't think this was the reason). The reason people used it was because it was the 'best' plane overall. It required very little skill to get kills in, it could take off from a carrier, and it could carry a fairly sizable load of ordinance.
It was better for killing GV's than any other plane, it was better for killing buildings than any other plane, and it was at least the equal of any other plane for killing other planes. It wasn't the BEST at killing other planes, but it was pretty easy, and that made it attractive in addition to the other reasons.
The Flakpanzer is the best at killing planes (again, with almost no skill required), the best at killing other GVs (see above note), and the best at killing buildings. Of COURSE people are going to use it... people being human, they look for the path of least resistance.
As I see it, there are two options. One, leave the Ostwind as is, and perk it. Two, make the other ground vehicles more attractive to use. This can be done in two ways. One, make the Ostwind much less effective against buildings and armored GVs. Or, Two, improve the other ground vehicles. For instance, right now the Ostwind is the best at killing ground vehicles, with the Panzer a very distant second. This could be resolved by making the Panzer do more damage do other ground vehicles. The Ostwind is also the best at killing buildings. This could be resolved by making the Panzer do much more damage to buildings.
I also think the Panzer's damage model needs to be looked at, as it has been shown that the Panzer really was NOT all that vulnerable to machine-gun and small calibre cannon fire. I don't feel like finding the posts where this was discussed, suffice it to say that it has been.
If the Panzer was made more attractive to use in an offensive role, we may see the pattern of a base attack consisting of 10 Ostwinds and 1 M3 change.
Until then, the Panzer will remain fairly useless, and the only vehicle that may as well be in the game is the Ostwind.