The top energy of the accelerator might not be a good judge of how practical its technology will turn out to be. The maximum center-of-mass energy of CLIC is 3 TeV [1], far in excess of what would currently be considered practical. Nevertheless the technology developed for CLIC is now being looked at for providing very high-energy electron beams for cancer therapy [2]. Making something like the FCC isn't just a matter of making a bigger copy of the LHC, it has its own specific challenges [3,4].

1 - https://en.wikipedia.org/wiki/Compact_Linear_Collider

2 - Slide 7, https://indico.cern.ch/event/578818/contributions/2487012/at...

3 - http://fcc-ee.web.cern.ch/content/accelerator-challenges

4 - https://indico.fnal.gov/event/16753/contributions/39405/atta...

Accelerator physics would have been unlikely to have progressed as far as it has without HEP. Beam line components for cyclo-/synchrotrons up to proton therapeutic energies are not seen in non HEP facilities (material physics).

FCC of course will not produce spinoffs that have already spun off, but what exactly will spin off we obviously can't predict very precisely.

I think you're engaging in a bit of cargo cult thinking here. FCC is analogous to early HEP, but it's an error to think that analogy implies similar outcomes. The sheer scale of FCC argues against similar sorts of direct spinoff. And indirect spinoffs are a poor justification for anything -- they can come from anything, after all, so provide no justification for a specific approach.