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Because making changes to "secondary" unit stats (like cooldowns, build time, abilities etc.) has proven to be a very good tool to balance units by adjusting their availability or timing, without having to nerf their performance. One major advantage with this is that it allows for units to remain strong to keep them attractive to use, but helps prevent them from being deployed in quick succession and/or in overwhelming numbers.
In the last 2 patches units like Pathfinders, Assault Grenadiers, Valentine and Jaeger Light Infantry all triggered a vast amount of complaints after their initial timings or performance changes about overperformance, and ultimately they have been fixed without touching their performance at all just by limiting their availability (I have barely ever seen any complaints about those units since the follow-up patches). These small changes are usually way more effective than people initially think.
As for the Churchill specifically, I personally do not believe a single Churchill is overperforming (except that grenade ability they had at vet 0), and that the unit is only really problematic when deployed in overwhelming numbers (~3 at once). So from my point of view, there is no reason to overly punish players using 1 or max 2 Churchills in a diverse build by nerfing the unit's primary performance.
So for now, availability will be more limited to see if that alone works, and if ultimately it doesn't work, performance can always still be nerfed later. But the first option is preferred.
I am very pleased to see that the MOD has taking into account the suggestion I have made many times to stoped overbuffing and overnurfing base stats and use secondary stats instead.
In the case of Churchill though there stat that are not inline with other units:
1) Rotation. Simply too high
2) Rear armour. Simply too high the value make PzIV unable to harm the unit even if they move in the rear.
3) easy access to defensive smoke
Suggestions:
Reduce rotation speed to similar levels with other heavy tanks
Reduce rear armour
Replace concealing smoke with smoke that provides "cover status" (not cover bonuses) or remove smoke or move to veterancy.