As a former experimental particle physicist, I don't think string theory or supersymmetry is going to be a description of nature. I say that despite spending a couple of years searching for SUSY. But I still think the cost of building a LHC++ (that is, the FCC or the SPPC) is justified. The current theoretical "dead end" or maybe swamp fundamental physics is in is precisely due to a lack of data.
Of course, there are a lot of smaller experiments that one could do, also in particle physics, that have great value. One example was the Muon g-2 measurement, another is anything neutrino physics. We can get a lot of interesting input and test our theories without going to the "energy frontier". Physicists understand this very well. But there are a bunch of questions that you can only really answer if you go to those higer energies.
As for the costs, 100 billion is a lot, but not when you compare it to other large infrastructure projects, or especially military spending (and note the Chinese proposal SPPC would be a lot cheaper!). I think if we could shave off 100 billion of military expenses and put it into basic research, it would be a great win for society. That's not realistic you say, with all the threats out there? Great, I agree, but now we have a nice project! Lets put as a common goal that we want to be able to do this kind of research, and then improve our societies nationally and globally so that we can reduce our military spending and do this kind of research.
100 billion ($/€) is the amount that Germany is spending as a special budget in response to Russia's aggression, in order to become the nation with the 3rd highest military spending on the planet. If it weren't for the war in Ukraine, we could put it all into such a project.
If we had averted COVID in it's early stages - like we did with SARS-1 and MERS - imagine all the resources we could have put into research instead.
