Dedicated "flat earthers" have already transcended that argument.

Firstly, they will point out the curvature still won't be visible from the jet.

Secondly, they will point out that the horizon always remains level with the eyes. If the earth truly were a sphere, the horizon should sink below your feet eventually.

Some even point to the private experiments done with weather balloons and GoPros, in which clearly no curvature is observable (besides that of the fisheye lens) even from > 30 km up.

And of course it is possible to go "around" the planet in a jet, but that would not be a convincing argument - how to discern whether the flight went around a globe, or in a circle on a flat earth?

A little background maybe: flat earth is supposed to look more or less exactly like the Azimuthal equidistant map projection of earth so well known from the banner of the NATO:

Antarctica is a wall of ice surrounding the waters and continents; the sun is an object only a few thousand kilometers away that draws circles more or less above the "equator" circle, which is why the middle and the outer rim are so cold.

I'm telling you, they have a few interesting arguments and it is a wonderful exercise for the brain to go back and wonder why exactly it is that we want to laugh them out of the room instead of addressing their arguments factually and calmly countering them with insight, knowledge and facts. I value flat earthers greatly, because they force me to read up on stuff I always took for granted, and they challenge me to re-evaluate my lazy "received wisdom" worldview.