I think you're heavily undervaluing manpower bleed.
In isolation, the Sherman is a better pick against the Ostwind than the Jackson. The Sherman will deal with the Ostwind and then contribute its hefty anti-infantry firepower to the battle. The Jackson will deal with the Ostwind, then do nothing until another tank shows up.
This applies more broadly: you want enough AT to deal with the enemy tanks, then stack the rest of your army with as much manpower bleed as you can. Investing in a load of AT you don't need is suboptimal play.
Tank destroyers are a reaction to the enemy composition. We see it so much because the dominance of the heavy tank meta since the Mobidef nerfs made it necessary: if you need two Tank Destroyers to kill a heavy, you're going to build two tank destroyers. And there goes 38% of your popcap.
Cutting the Tiger down to 960 makes one tank destroyer that much more able to deal with it, freeing up that popcap and those resources for other vehicles. It might, ironically enough, reduce the number of 60 range TDs fieled.
Completely disagree.
Provided you don't know what your opponent is going to build, an M36 is
always the best choice.
Let's take your Ostwind/Sherman example. The Sherman does give more AI power, which is nice, but is vulnerable to any other OST vehicle from T3, or a panther, or most call-ins. Meanwhile, the M36 isn't vulnerable to those vehicles (when correctly micro'd).
As a result, the safest option is to preemptively build an M36, since this will invalidate
any vehicle OST goes for, forcing them to either heavily invest in AT-focused tanks (STUGs, Pathers) or more heavily invest in AT-Infantry. For the former, the solution is simply
more M36s, and for the later, your infantry should counter them.
The current Tiger HP gives OST a generalist solution to the problem, since it has enough HP to somewhat resist TDs, but also allows for the vehicle based AI that OST needs to survive late-game. Nerfing the Tiger's HP removes that, forcing the situation I described above.
/edit
I don't say this lightly, but the
only solution to the current "60 TD Meta" is literally a rework of every tank in the game. We're currently stuck in a situation where either "60 TDs" are the obvious pick in every situation, or they're nerfed to the point where the game is unplayable. This
needs to be fixed; and the only way to do this is by completely reworking how every vehicles relates to each other.