Do the two points have to be on land? I would think since you can see the ocean from Aconcagua you'd be able to see all the way to the horizon and that would be the longest sightline

Out of the question. The higher up you are the farther the horizon, I wouldn't be surprised if the longest horizon is from Aconcagua. I have been at 18,000' on Kilimanjaro, the view was absolutely stunning. (But we were tourists, not mountaineers, and had no traction gear. At the time that meant progress became impossible very soon after sunrise. Now the ice has retreated so far that you can expect to summit by daylight without traction gear.)

But the longest possible view isn't to the horizon, it is to another point that can see the same spot of horizon on a reciprocal bearing. And ocean has no such points.

I think the main insight is that the ocean horizon arrives quite fast.