Really many people are indeed involved. On the physics side, this is testified, for instance, by the number of authors of this FCC-ee and this FCC-hh publication (yeah, I signed both ;) ).
The age of stumbling across gold is over!
This is true. With the LHC, we knew in advance that either the Higgs boson would be found, or we will have to revise the Standard Model (in the no-Higgs case). So that we have a no-lose theorem: there will be something to learn.
Now, the picture is different: we have no certainty of anything. We all rely on how kind nature is with us, somehow ;)
Other than for pure knowledge sake, what are some applications of the information that may come out of this project that would enrich everyday life?
Pure knowledge is very hard to give any value, although it is really valuable. There, this should be ignored from any reasonable cost-benefit analysis. Especially as a new understanding of the universe functioning won't change our lives in any sense.
The immediate value (let's call it that way) is elsewhere.
First, there is a scientific value (the publications that will stem from the machine).
Second, there is a human capital formation value: all early-stage researchers that will be trained on the project may go later into the private sector. It has been demonstrated that their salary is generally higher when compared to people of the same age category not being formed by research (cf. the LHC).
Third, many private companies will be involved in the development of the non-existing technologies required by the project. This is the somehow 'learning by doing' thingie.
On top of that, we have the transfer of knowledge from the public to the private sector. For instance, many pieces of software have been developed by particle physics research and are now used everywhere (not even speaking about the web itself).
Finally, we may even add the outreach activities as these can be converted into a benefit, somehow.
I know this is not a full analysis (this may go into a full post if I wanted to give more details), but I hope this will give you (and possibly @dhimmel) enough insights!
Those are all phenomenal points! I mostly asked to get your take on it, as I agree that no cost-benefit analysis will capture the positive outcomes of these two projects. Thanks for sharing and may leptons collide!
Hopefully I will see that in my researcher lifetime :)