"As for applications, my position has always been that if there were zero applications, it would still be at least as scientifically important to try to build QCs as it was to build the LHC, LIGO, or the James Webb telescope." makes no sense to me. All three of those were well-understood and known projects with specific goals. If there are zero applications, it's not as scientifically important to fund QC-building labs. Note that LIGO had no real application (in a product sense) - but it existed to answer a well-defined long-standing question of importance to physics. QC isn't going to solve any important scientific problems even if we build the next generation of them.
I definitely think QC will answer a ton of important physics/quantum mechanics questions. Worst case, if research into QC ends up proving that no feasible error correction strategy can overcome the inherent noise, that would be a major advancement. At present, QC is more promising than that. Scientifically it’s comparable to XENON, LIGO or the LHC. The worry is that from a market perspective QC might turn out like nuclear fusion.