Tanks are pretty expensive, but well worth it if you can acquire large droves of them. And by large droves, I mean over 50% of your army’s IPC value, and only if you are clearly on the offensive. To be able to attack an enemy’s major stack, you are definitely going to need a larger value army since infantry are still very efficient on defense. If you go with the 1 tank=2 inf comparison, let’s look at a couple scenarios of an attacker with 150% value over the defender, with LL for consistency.

Straight inf:

30 inf vs. 20 inf—> 20 inf/10 inf

no counterattack

20 inf vs. 10 inf—>15 inf/4 inf

no counterattack

15 inf vs. 4 inf—>13 inf Remaining attacker value: 39 IPC

If the attacker has 2/3 value in tanks:

10 inf, 10 tank vs. 20 inf—>10 inf, 10 tank/14 inf

14 inf vs. 10 inf, 10 tank—>8 inf/5 inf, 10 tank

5 inf, 10 tank vs. 8 inf—> 5 inf, 10 tank/3 inf

no counterattack

5 inf, 10 tank vs. 3 inf—> 5 inf, 10 tank Remaining attacker value: 75 IPC (72 if defender counters the second time, but then you get the TT sooner)

You end up with nearly double the remaining unit value left. However, the weaker defense means the defender can attack first to kill a bit more:

20 inf vs. 10 inf, 10 tank—> 13 inf/3 inf, 10 tank

3 inf, 10 tank vs. 13 inf—> 3 inf, 10 tank/ 9 inf

9 inf vs. 3 inf, 10 tank—> 3 inf/1 inf, 9 tank

1 inf, 9 tank vs. 3 inf—> 1 inf, 9 tank Value: 57 IPC

But you still don’t have to replace as much. Finally, tanks allow multinational armies to work very nearly as effectively as single-nation armies. Say France has a similar value army compared to Germany, but is comprised almost 100% tanks, and Britain/America have >50% value of France’s army in infantry, it can function exactly the same as the scenarios above, because the attacker takes on average 0 casualties, and the allied infantry can just move in after the initial attack is made to defend the tanks. (The supporting infantry won’t be able to attack though, so each tank attack will do slightly less damage and require more defensive support to discourage enemy counterattacks)

Of course, these scenarios ignore artillery, so a mix of infantry and artillery will probably take fewer losses than straight infantry, but they will never be able to completely nullify offensive losses like tanks. And unfortunately, games are probably decided by the time someone acquires this many tanks, except maybe the Brits finishing the Ottomans.