Notably, Mercury was not a motivation for the development of relativity. Even with the benefit of hindsight, I don't recall ever seeing anyone critisizing conte

Notably, Mercury was not a motivation for the development of relativity. Even with the benefit of hindsight, I don't recall ever seeing anyone critisizing contemporary astronomers for looking for the missing planet. GR was developed based on theoretical concerns. The fact that it happended to solve an open issue of Mercury's orbit was a nice confirmation that it is on the right track. The good news is that we have a lot of physicists looking for an alternative theory of gravity called quantum gravity. It would be great if they come up with something that solves the theoretical problems with GR and happens to resolve dark matter.