It's absolutely true, and the only thing I missed and you are not wrong about is that supplying armies via ship was possible, if they only operated near ports. That's exactly why the Roman empire conquered lands around the Mediterranean but never advanced deeply into central Europe or the middle East.
But no, neither depots nor wagon trains can supply a large army over long distances. It's not too expensive, it's impossible, because the animals that draw the wagons (and build the depots) themselves eventually consume more supplies than they can draw.
A force of 2000 men over 100 miles? Yeah, that may be in "doable, but expensive" territory. 50,000 men over 500 miles? Forget it.
They did conquer the entirety of Gaul, which is not just coast. And France isn't small. Romans built roads, supply depots, carried supplies on pack animals.
And if you think you can move 50,000 soldiers 500 miles deep into enemy territory without supply lines, you can forget about that too. Napoleon did use supply lines from Poland into Russia. Remember: we're talking about Sun Tzu's advice not to bring your own food but take it all from the enemy land, and I'm saying that doesn't work in all circumstances. I'm not saying it never works. But there are very good reasons why historically, large armies did rely on supply lines.
But no, neither depots nor wagon trains can supply a large army over long distances. It's not too expensive, it's impossible, because the animals that draw the wagons (and build the depots) themselves eventually consume more supplies than they can draw.
A force of 2000 men over 100 miles? Yeah, that may be in "doable, but expensive" territory. 50,000 men over 500 miles? Forget it.