Protontherapy alone is enough justification in my opinion. Add the many kinds of detector offshoots that find their way to industry. The web itself, if you wanna go back a bit further. Aren't those worth it you think?
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.
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...