|Guillotine|
Chieftain
The AI in my experience (I haven't done multiplayer) just sends masses of units. It doesn't have cavalry on the flanks to stop your attempts to flank him. It would be easy enough for humans to counter (though still annoying) but it would be easily exploitable versus the AI, and most games are versus the AI. Even assuming it is given ways to deal with it, put even one spearman and one cavalry unit next to that transport and the enemy is going to have a tough time destroying it before you deploy your instant army on their flank.
For chokepoints: you asked "In real life, what real chokepoint do you see that entire armies are forced down to let maybe 1/20th of their army go in a line?" In my example, the Persians did try to flank them with their navy, but their land army still needed to go through that pass. In mountainous areas, that's pretty common. In the fighting in northern Italy in WWII both sides tried to ambush the enemy when they were going through those narrow passes. If you need to get somewhere, and the only way is through a narrow chokepoint, trying to go through isn't necessarily a strategic blunder.
Back to transports: yes, humans do tend to find new ways to solve issues. However, they really did take thousands of years to discover a better way to move armies around. Putting people in a siege tower or battering ram may work in a siege because there it protects your troops (and you want these transports to be easily destroyable). It is completely impractical for attacking other countries as a whole.
Ahh well seeing as the A.I. has terrible pathing, and unit control as is, they already need to Completely reprogram it IMO. And if your protecting it as so, the troops couldnt deploy anyways, making over protection worthless. It has its ups and downs. Fighting the A.I. is an exploit against the A.I. already. They suicide units, make dumb decisions, and just down right play stupidly.
But you could just as easily find a way around that pass. Perhaps sail south, or north and find a way to come around. I would NEVER try to take a small pass from an enemy, no matter how many troops i have. Its suicide.
Well, if they were to add in those, i would see them not easily destroyed. Thats a different all together. Every army, even in early times, would move to a staging point before preparing for war. Not leave there home cities dressed for war, marching sword in hand. They may have marched on foot, but their supplies were carried by other means. I think it would just be something to look into. Even if you couldnt use it till later in the game.