1.) It's well known that ambiguous curvature on a RE wouldn't be apparent until 60,000 feet in altitude.
"Well known"? It's already been established on these forums that popularity!=truth (popularity does not equal truth, for the nonprogrammers

). Besides, it can't be that well known, because most of the REers on these boards say otherwise.
2.) A wide angle lens can produce a curvature of field effect. As could Snell's law and the sphere of vision effect.
Which would also distort the plane.
3.) That horizon is much too foggy to discern any sort of curvature.
I'll admit, this picture can be ambiguous.
4.) I've been to 45,000 feet on an international flight and found no curvature when comparing the horizon with a straight edge. Neither could the passengers around me.
Of course you wouldn't. You just said there wouldn't be any apparent curvature until 60,000 ft! What happened to the fisheye lenses in the windows, Tom? Is the conspiracy really that hit-and-miss?
5.) We have a pilot on this forum who says he has never seen curvature.
So we have one person's word that there is no curvature. Correction: that he has not seen curvature. What happened to all of the phenomena that you just threw at us? Is nature as inconsistent as the conspiracy apparently is?
6.) Photographs are not admissible as evidence on this forum.
Unless they could prove an FE.