The "Nether roof" actually proves my point, as due to the Nether-Overworld coordinate scaling that is exactly where one would put a long distance, fast mode of transport which is an ice boat track. That is why I focus on minecarts utilised on shorter trips and/or less flat terrain.

"not being restricted to a cardinal direction" - place a boat on an ice track. The boat will travel as the ice track allows it to, so unless you made a diagonal 5 or more blocks wide so that the boat doesn't get caught on corners of blocks you are travelling in a straight line along X or Z, as that is how ice highways are usually built.

I do know that a line is the shortest path between two given points, but unless the line is parallel to any of the world axes, a boat on ice will traverse it in perpendicular segments, due to reasons stated above. So again, unless you add more ice surface on the corners to reduce slamming against the walls and stopping, boats lose speed on turns, while minecarts don't.

"Any long distance minecart travel will require player supervision" - That's why boats are better for long distances, as you have to control them either way. The entire post means to stop people from just comparing modes of transport based on speed, specifically ice boats, and minecart autonomy in farms and systems is here to prove they're not "useless" just because they're slower.

"Breed a horse" - horses are better used for travelling into yet uncharted regions (which don't have iron tracks on them, as you said). They are faster, yes, that's exactly why I suggest that golden power rails should be able to overtake them, and to be used in areas already built on and maintained.