Research is never a sure thing, but ideally we'd push all areas of science equally. How many other areas of science have a $100B tool that may or may not reveal something interesting?

Why should high energy physics get priority over any other area?

It's the nature of high energy physics that the higher the energy the rarer the event. The standard model already explains our universe unbelievably accurately. Sure there are corner cases, and maybe, just maybe a LHC++ might give some evidence towards one model or another. But what happens if global warming, wars over fuel, lack of fertilizer, and lack of food makes it harder for humanity to survive let alone complete major science projects.

Renewable and/or carbon neutral energy in all it's forms seems pretty important these days compared to slightly reinforcing one physical model over another.

Maybe a fusion engine would let us explore the solar system enough to find planet 9, which might well be a black hole. That would open up particle energies well in excess of anything we could manage with LHC++.

That was a small part of my argument. That you have to concentrate on that in this way shows that you don't really have any arguments yourself.

What is meant by "solving fusion" if not making fusion an economically viable source of power?

And I didn't mean to suggest that available energy was the limitation on building LHC++, but rather that available economic surplus was.