Luck plays too big of a part in the hit/miss chances for your above post to be entirely correct. Forty 5k tanks and a hundred 2k snipers equals about 400k HP in total. Take into account that most tanks are going to have around the 7-8k mark, so I'll just go with twenty 10k tanks and twenty 5k tanks plus your hundred snipers. This means more realistically the total is going to be ~500k. Against a defending side of 120 people and assuming that about two thirds of them have LOS and about a quarter of those shooting miss their shots, you've got about 60 people doing an average of, say, 300 damage each (yeah, this is a battle without those stupid golden guns). This means a total damage of 18k per round which means it would take approximately 28 rounds to kill the entire attacking side. Note that this is assuming the attacking side is just standing still on the ground without mounting any towers or doing anything to avoid LOS.
Now, if we calculate your 140 tanks using the same theory as last time (seventy 10k tanks and seventy 5k tanks), that equals a whopping 1.05 million HP! Also using the same theory as above, the defenders, doing their 18k damage per round, would only be able to eliminate the attacking force in at least 58 rounds! And since fort battles only last 55 rounds, anyone who can manage to amass 140 5k+ HP tanks would be nearly unbeatable. Again, that's without the attackers mounting towers, avoiding LOS etc. (not that they would need to when they could rush the flag every battle without taking but a scratch to their HP percentage).
Not to mention the opponent's numbers would be decreasing as well so their damage per round would grow gradually smaller, thus making it even harder to kill the monster army.
Also remember that some of those 5k tanks would be adventurers. I don't even want to think about the results.
I know that the numbers are not going to be exactly as I have stated above, and I also haven't taken into account dueler critical hits, but you can lower the HP amounts and it would still have the same result on a smaller scale.
Like I said above, the hit/dodge chances are too random for a player with wicked fort skills to pose any significant threat to a HP tank. You speak as though HP tanks can't dodge and can't hit anything. My 7k tank has a personal best dodge record of 109, and I am usually towards the top in the damage category.
I'm not saying tanks are the only way to go (really anyone who knows what they're doing can make an impact on a battle starting with 1.5k HP), but your argument that tanks are not as good as "snipers" just does not hold up when faced with any sort of objective scrutinisation.