There are some excellent answers here already, and I won’t try to compete directly with their explanations. However, your comment about “free lift” from a headwind was an important clue about your thinking, and I wanted to take a slightly different approach in my answer. (A little more “touchy-feely” perhaps...)
What is apparent is that you are looking at this from an earth bound frame of reference. Because a kite, tethered to the earth, DOES extract energy the energy from, and experience wind as “free lift”. In response it climbs.
I know that intellectually you understand that airplanes are not tethered, but try to find a way to untether your mind from this groundling paradigm. Aircraft are literally “craft of the air”. They don’t know or care which direction or how fast the surface of the planet below them might be moving, they only respond to and interact with gravity, plus the aerodynamic effects of movement through the viscous three dimensional medium that contains them - The airmass.
Only the pilot cares about the surface of planet below, and only to the extent that he/she desires to interact with it. (I.e. navigate to a place on it, touch the wheels to it, avoid hitting protrusions of it, etc.)
No “complicated numerical analysis” is required, just a gentle shift in mindset. Augmented by thoughtful observation of the world around us. Do you have a river nearby? Go to it and quiet your mind... Watch objects float by. (If nothing is floating by, throw a stick in). Observe how floating objects are one with the current.
If it is safe to do so, go swimming. Notice the effort needed just to keep position with a spot on the bank? Relative to the shore, can you swim faster upstream or downstream? Do you get any “free” energy swimming upstream as you presumed an airplane might get from flying into a headwind?!
Maybe you don’t even need to go swimming, hopefully you “get it” now, so I won’t belabor any further... ;)
Good luck with your upcoming lectures.