Because there would no reason to wait for the CP and invest on tech to get heavy tank otherwise.
When you lose a tank, you lose/engage
1- the initial investment cost.
2- the potential replacement cost.
3- the value you get from it (firepower etc...).
4- the veterancy.
So if 2 mediums = 1 heavy then if you lose 1 of your two medium tanks you lose/engage half each of those 4 points. But if you get your heavy tank to half life you lose nothing from them.
1- You still have your investment available.
2- You don't need to replace it.
3- The value you get from it is still fully available.
4- The veterancy isn't lost and rise.
So nope, even if you get to lose something from having your heavy tank taken to half life you'll still get advantage from it.
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The more economy you inject into the game the more it beneficiates to durable units like heavy tanks because their only CONS are their initial investment unless you lose them which is quite difficult and requires a lot of investment from your opponent, this is why cooldown were added to their replacement cost.
At the beginning heavy tanks durability was offset by heavy infantry management cost, but with all the cost reduction applied to many infantry units over the patches they became just predominant and over shadowing everything else because of the amount of resources available.
That's why I think that instead of reducing the player income, there is a huge need into increasing the infantry management cost.
That would be beneficial for the balance because it would reward good play and preservation, be a nature CONS to infantry blob and spam, make light, medium tanks and doctrines without heavy tanks more appealing for their timing.