From a non-expert view with no influence on the matter in any way, I do find these sorts of arguments more compelling than anything I've seen put forward by proponents. "It will indirectly help with superconducting magnet tech" is a variant from the proponent side -- I even like this one, because it's needed for fusion. But ok, can we just fund that tech directly, with or without the purpose of enabling fusion plants, and not as pork (even if fundamentally necessary pork) to this collider project that needs to justify itself on its own merits?

LHC reaches collision energy environments in the scale of 10^4 GeV = 10 TeV, the FCC proposal is shooting for 10^5 GeV = 100 TeV. Just one order of magnitude higher, and note this won't appear any time soon. The LHC itself took 10 years to construct, another 4 for the Higgs detecting experiment. The FCC would probably take at least that long, I've seen estimates of 20-30+ years. Is this really the thing to commit on? It's not just money and time, but also attention. The thousands of scientists involved in particle physics are smart people, is a bigger collider project the best way to use their talents, for deep probes into reality or otherwise?

I don't know what's required for even higher energy levels -- like what would an equator sized one even be capable of? A quick google suggests with 30 T magnets, around the order of 10,000 TeV = 10^7 GeV would be achievable. Still a far cry short from the >= 10^(11±1) GeV level estimated to be the point where electroweak vacuum stability breaks down, and similar very high levels suggested by grand unified theory topics. Maybe decade+ projects should focus more on figuring out how to build (probably linear) accelerators in space to reach much higher energy levels than just 100 TeV? (Of course the max proton-proton collision energy isn't the only design parameter.)

The lack of "mandate" is of course striking, too. Papers from the 90s related to the LHC proposal all mention the importance of the Higgs and how various calculations let them be confident the LHC could find it (or not find it and invalidate so much). But, to try and credit the FCC proponents, it's not like you can't easily find some justifications. Whether they're convincing to different audiences is another matter. Several years ago there were a series of 5 papers on uses of the FCC, to be turned into the current 4 volume CDR. Here's the one (a couple hundred pages) specifically on new physics: https://arxiv.org/abs/1606.00947 There's some classes of dark matter theories it could rule out (but not all, so ultimately a similar situation with supersymmetry), and the paper helpfully compares reach vs. the LHC.

In the end I'm not even actively opposed to the FCC proposal, I just don't think it's adequately justified itself on its own merits or in comparison to potential other uses of funds and attention. (The latter being somewhat important because, while it'd be nice if there were more budget and more people, budget proportions probably aren't changing anytime soon, and birthrates are declining.) That's fine, what I think doesn't matter, and many things get funded that don't really justify themselves either. It's just nice when the benefits are much more direct and more easily explainable to non-experts.