But then you still dont have any additional incentive to get an ostwind anyway once you get your t4 up.
The whole point of tying the ostwind to t4 was to shutdown the okw scavenge callin build. 2 trucks and 6-8 cp is just asking for that back. Everyone will do mech into battlegroup into ostwind spam again, effectively killing all build diversity. As someone who used scavenge both before and after the patch, scavenge is a lot more healthy for the game and interesting to choose in the current patch.
Its cool and all to propose a bunch of changes so you can contribute ideas, but changing things for the sake of changing them is not the way to go.
Overly powerful callins are indeed very dangerous for build diversity. That is a given. The problem is though, that current solution, which is tying them to tech limits build diversity as well. It does it by definition and you cant tie to tech without limiting the number of builds. This is against the idea of callin that is supposed to increase build diversity by offering a slightly weaker or specialised unit for the benefit of not having to tech.
That is why I think the current solution is bad and there is a need for another one. The new solution should allow players to build doctrinal units at the moment specified by CPs and without having to tech to overly expensive late tech. Still, it should add some cost to building a callin itself, so you are not given a early tech and callin unlock for the same resources.
What I can suggest as a first idea is to add upgrade to the HQ, say for 200mp/60fu (or more), that would unlock vehicle callins in commander as long as you have required CP. That way, if you want to get for example ostwind and conserve fuel for the spam, you wont be given ability to build units from one of the early trucks. Same for usf: if you want to go m10s, its ok, you don't pay for major, but you either pay for 2 officers and upgrade which is already as much as 1 officer and major, or you skip one officer, so you have less choices when it comes to building support weapons.