Your reasoning applies equally to airports. Congrats, you just "debunked" the airplane! (duh!)

So even if you don't want them to, people can drive to near an entrance and park. There will be induced traffic

This is vacuous. There will always be induced traffic for such a project. But the kind of planning required for the kind produced by an airport is materially different from the kind produced by an entirely new highway. Also, if your new "airport" requires the construction of a new road in the future, this isn't as big a short term barrier as those around an entirely new major highway, and will be borne by the local government.

FWIW This also means you have to ban internal combustion engines

FWIW, I'm glad you finally caught up with my actual idea!

You don't even know what "all other" is, do you?

That's not relevant at this level. I don't have to know all the details of building a road to know that not building a road and flying a cargo will avoid those expenses and incur different ones. What is your point, here?

(Here's a hint, when discussing major infrastructure, don't automatically ascribe to others straw man positions of zero cost! That's not exactly a "charitable" reading.)

And I'm as ignorant as you, but

...a little soft in the reasoning. If digging costs are exponential with depth, then this would be an interesting point. I'm not sure if I should really credit that. Well, I guess one would deserve credit for bringing that to attention. So your comment makes, charitably, a "half point."