To me, bringing up that the WWW, fancy PET scans, etc. came in part from HEP and therefore we should be grateful is missing one major point. In the WWW case, in an alternative history it seems likely to me that some other project would have developed something similar around the same time if the funding were allocated differently. As for the other benefits, I think if the money were allocated towards research in those areas it would have been more effectively spent.
Ultimately I think the worst "cost" for HEP in general (not just colliders) is that it encourages something like brain drain on other fields. I think that people interested in certain mostly non-practical areas of physics like HEP would do the world a lot more good if they instead got a PhD in some field of engineering or CS, or at the very least a more practical area of "physics". Here I refer to physics in the descriptive sense which doesn't align well with the prescriptive definition of physics. E.g., I don't understand why fluid dynamics is not "physics" in that it's rarely researched in physics departments (unless studied indirectly in chaos theory or plasma physics). Must be too Newtonian for people to care. (For what it's worth I'm at the end of a PhD in mechanical engineering studying fluid dynamics.)
