Couldn't the LHC be upgraded then to take advantage of this? Sounds like a much cheaper option. But I'm not a physicist at all so I'm sure I'm oversimplifying.
The fundamental limitations isn't the detectors or computers, it's the energies you can reach with a collider of that size. Yes you can completely retool the LHC, which will probably allow you to do slightly better precision measurements, but won't strongly affect the energies you can probe.
To get to really high energies you need to build bigger. As a (former) particle physicist, I am also a bit sceptical if going bigger is better; the LHC was a calculated risk, with the goal of finding SUSY, which was a very plausible theory at the time.
The FCC seems to be a shot in the dark.
There is the argument that if we don't build a bigger one now we'll lose a lot of expertise, but that feels like a knee-jerk reaction from the community.
Superconducting magnets enable much higher energies in smaller (granted, theoretical) fusion reactor designs than previously possible. Does the same not apply to colliders?
It's a valid concern that must always be considered. It's the kind of thing that's been biting Intel in the ass for the past 7 years. Even if your decision is "we'll lose a lot of expertise and that's okay", it needs to be a conscious, well-grounded decision, otherwise that is a knee-jerk reaction based on your own biases and assumptions that could have significant, even irrevocable, consequences for the future.
This is effectively a measure to maximize the output we can get from the machine, so to maximize the return in investment.
There’s a lot of debate in the field what should happen next. FCC is one possibility, but I’m personally sceptical (even though I work at CERN myself).
It seems to me there’s a clear need for a Higgs-factory as a follow up to LHC, but that doesn’t have to be a ring collider, a electron positron linear collider could fulfill this purpose as well.