> The hypothetical machine could not truly test string theory

Interesting point, there was a time where it wasn't even clear whether string theory should be considered part of Physics or rather Mathematics or even Metaphysics. That said, the LHC has continuously given additional insights into existing theories and also it's not the only collider in the world although probably there's no other where so many countries collaborate. Also an interesting read about the early history of CERN: https://physicstoday.scitation.org/doi/10.1063/1.1955503