The first argument that comes to my mind is that General Relativity is a very beautiful theory. By the time of Newton the scientific establishment was already fixed on the idea of a spherical earth. If this was a massive conspircacy then it would be very weird that, after the conspiracy had already fixed which hypothesis it was going to favour, Einstein found an elegant piece of mathematics that was consistent with the hypothesis that they had happened to pick.
The first argument that comes to my mind is that General Relativity is a very beautiful theory. By the time of Newton the scientific establishment was already fixed on the idea of a spherical earth. If this was a massive conspircacy then it would be very weird that, after the conspiracy had already fixed which hypothesis it was going to favour, Einstein found an elegant piece of mathematics that was consistent with the hypothesis that they had happened to pick.