I find it fascinating that the prevalent comment on this are variations of "that can't be right, we still don't understand [things, such as dark energy, etc]." It demonstrates one of the most pernicious biases in science.

Particle accelerators have been the mainstay and principle tool for particle physics, and have produced amazing results. Further, there are obvious questions in fundamental physics that the Standard Model revealed by these experiments does not answer. But, it does not follow these further questions can be answered via accelerators that us humans can construct. The universe has made no promise to us that its mysteries are accessible, least of all through some particular method. We had a particular reason to believe the LHC would reveal new physics (the Higgs boson prediction) and we have no such reason for the proposed future accelerator; our evaluation should change as a result.

My guess would be that astrophysics is a better route to understanding the mysteries of fundamental physics; astronomers can measure distant phenomena with surprising accuracy already, and objects such as black holes are one of the few cases where exotic phenomena are currently widely expected to occur. Perhaps the money is better spent on a truly gigantic space telescope?