Yes, using lat, long, and radius is better than xyz on the earth - usually. But the radius, the 3rd dimension, changes by where you are because the earth isn’t actually a sphere. On this path it would get longer until you reached the equator, shorter until the most southerly point then longer again until you hit NZ. It is a wavy line not a straight one. Again you’re projecting 3d onto 2d (because you’re incorrectly assuming a fixed radius) and saying that that error in projection doesn’t matter.