First off, tanks were always scarce on the Pacific side of the board because of the island hopping thing, and they are especially scarce in 1940 because of the fact that you can’t go into Russia, so there is almost no reason to use tanks since the largest open area of land is China where they can produce anywhere. In Europe where you have production facilities and much more contiguous land area, tanks have always ruled. Tanks will still rule the battle for the Eastern Front, however, their numbers will be fewer because of the mechanized infantry.
I think the major issue with this analysis is strictly comparing tanks ONLY vs. mech ONLY. In reality the real use is going to be a combination of mech with tanks, which is precisely all the more reason that tanks need to cost 6 now, because now we have a unit that can move two spaces AND blitz when paired with tanks, besides all the tank’s other abilities, so mech actually make the extra cost necessary. Tanks have been made more valuable in these 1940 versions, which means they need to cost more.
If tanks still cost 5 IPCs with the introduction of mech at 4 IPCs, you could buy a mech and a tank with the same 9 IPCs you could purchase 3 infantry with:
3 infantry vs. 1 infantry
Win: 90% Lose: 8.3% Both Dead: 1.7%
1 mech, 1 tank vs. 1 infantry
Win: 89.7% Lose: 5.1% Both Dead: 5.1%
With this setup, you have just about the same chance of winning, plus a decent chance of still destroying your opponent even if you die too, but the real kicker is that you can move BOTH these units 2 spaces at a time! Increasing it 1 IPC keeps infantry from dying to the mech-tank combo, but doesn’t kill tanks either, as I’ll explain why next.
Forget that you can buy mech at a 3:2 ratio to tanks, you also have to look at the fact that you can still buy tanks at a 5:3 ratio to fighters, even with the cost increased to 6! 5:3 is a bigger ratio difference than 3:2 (you can get 1.7 tanks for every 1 fighter, while you can only get 1.5 mech for every 1 tank), so if you’re going to be buying fighters to combine with your mech instead of tanks, you’re going to end up on the short end of the stick, especially when you factor in AA guns as gamerman mentioned before. Tanks still hit on 3 when attacking, just like fighters, but cost a lot less and provides immediate defensive support to your frontline mech that planes can’t. This can be especially dangerous to an opponent if they leave a blitz path open, and while you say this is rare, that is the case in previous editions because of tanks. Tanks force your opponent to close holes and defend territories they otherwise wouldn’t have to. So tanks are still going to be necessary as the only unit that can threaten a blitz and a unit that can capture territories on its own and then defend them, which fighters can’t do.
You can compare mech to tanks straight up all you want, mech obviously win; but most players are not going to be buying ONLY mech instead of tanks, just like buying ONLY infantry (just because the math says they’re the best on defense) or ONLY tanks (just because the math says they give you the most punch and movement per IPC) in previous versions wasn’t the best idea either. Neither are fighters dead because tanks have a better ratio, because they are still the most versatile unit. You have to consider the big picture and not just compare mech to tanks, but combinations of the two to other units, especially your mech-fighter combos to mech-tank combos. Your numbers are correct, but your focus was just too narrow.
The rumors of the death of the tank have been greatly exaggerated. Hopefully this math helps you see that. :-)