I think theres decent motivation behind this conspiracy. If space missions weren't actually real you would be able to spend the billions that go to space missions on any other sort of nefarious thing.
But the evidence is damningly clear that Earth is a sphere lol
One of their arguments is that if you were to fire a cannon straight up, the ball would travel from the original point because if the earth is round then it would spin while the ball is in air.
another is because water will always lay flat on any surface, if the earth is round then the water would all float off into space.
OH and in order for a plane to keep an elevation of 16,000 feet or whatever, they would have to be constantly adjusting their pitch downward or else they'd fly off into space.