It doesn't feel ambitious enough to me. Spend 10s of billions to get a less than 10x energy about 40 years after LHC reached its peak. Also planning of the next one should have happened while the last one was being commissioned to avoid the huge gap between them.
The link mentions 100 meters depth. Without being an expert that sounds deep enough to withstand hurricanes and such.
However 100m depth? For a structure _that_ size, which I think might contain high vacuum, let alone systems that might need humans to service when they break? What is the feasibility of that problem?
As mentioned in the linked presentation, they envision using interconnected 300m (1000ft) long segments that can be changed using ROVs, using technology that's already available for underwater pipelines.
Also mentioned is that the LHC cryostats, some nice illustrations here[1], are very nearly neutrally buoyant. Given these would be built similarly, they wouldn't need significant infrastructure to keep them in place.
So no humans would need to go down there, if something breaks they could just replace the broken segment(s) and fix the broken one(s) on the surface.
"So no humans would need to go down there, if something breaks they could just replace the broken segment(s) and fix the broken one(s) on the surface."
This frankly is not possible. Colliders need constant maintenance. The "Collider in the Sea" is not a real proposal.
Presumably the plan would be you're actually building a the collider tunnel much like CERN under water, so effectively a very long pressure structure. This...probably isn't super-unreasonable for a static structure where you avoid the cost of excavation - i.e. it depends on how different in diameter from something like the Nordstream pipelines you'd be going.
The first unfounded assumption is that, regardless of what the LHC found, there was always going to be a need for a larger collider of the same design. This is clearly false - the design of the FCC (for better and worse) is specifically geared to confirming or probing various discrepancies that the LHC found, and to finding things that the LHC was hoped to find that didn't pan out. If the LHC had found SUSY partners or WIMPs, or if it found no conflict with theory whatsoever, the next accelerator might have had a radically different design.
Secondly, you're assuming that it's possible to build a collider that has (significantly) higher power than that with foreseeable technologies. The reality is that we are very close to the limits of what seems achievable in terms of particle accelerators, and per the people at CERN, even this 6x increase in collision energy will require ~40 years of research to be achieved. And the collider we need to probe regimes where we really have good scientific reasons to believe will reveal new physics (i.e. the regimes where we know the standard model breaks down) is not 15x or even 100x higher: even a collider buried around the entire equator wouldn't get anywhere close, we'd need a collider the size of the solar system to actually probe these regimes. So going 6x or 10x or 50x is irrelevant, unless you're looking for some very specific technicalities, like they are at the FCC.
They did start planning around then. I don’t remember the exact date of when the FCC working group officially convened but they’ve been working on it at least since I was a grad student in the early 2010s.
