You make good points, but I think you should revisit your assumptions.
Fission is so expensive mainly because of safety and regulatory concerns: radioactivity scares people, and so safety requirements are cranked up to 11: everything is overdesigned just so that current regulator is confident enough (next one might one up anyway), lots and lots of concrete and steel is used for containment etc. Additionally, the whole construction process is hopelessly bureaucratic and takes forever, requiring inspections of inspections, redesigns for things that in, say, coal plants would just be redone differently by contractor on site and so on. We most definitely could make fission significantly cheaper, and in fact it has been so in the past.
Now, fusion does not have similar safety requirements. If the plant blows up for some reason, there's no significant radionuclide contamination. Therefore, there's little reason to require safety standards significantly higher than those for coal plants (the boilers of which can also catastrophically blow up). This makes it significantly cheaper, due to reduced material use and construction time/labor for containment, and reduced regulatory delays.
Your point about power density is also good, but I don't think it matters very much in practice. Sure, fusion reactor might have significantly lower power density than fission reactor, but so what? Renewable plants are even worse when it comes to power density, yet it doesn't stop them from being rolled out. We don't lack space for fusion plants, even if they are much larger than equivalent fission ones.
Fission is expensive because all the components have to be highly reliable, for safety reasons.
But in a fusion reactor, all the components also have to be highly reliable, because they will be so radioactive that repair will be difficult or impossible. The stuff that's hot in a fusion reactor will be far more complex than the stuff in a fission reactor, which is technically very simple.
The reliability is costly, regardless of the reason for it.
Of course power density matters. The cost of manufactured items is related to their size, and fusion reactor cores will be far larger than fission reactor cores (as well as being far more intricate). Are you saying this 10-100x larger size will NOT be reflected in the cost?
Renewables have lower power density, but also allow omission of entire parts of a thermal power plant. No steam turbines, no heat exchangers, no cooling systems. They also tolerate far less reliable components, because failure affect just a small part of the overall output of a field of turbines or PV modules.
Fission is so expensive mainly because of safety and regulatory concerns: radioactivity scares people, and so safety requirements are cranked up to 11: everything is overdesigned just so that current regulator is confident enough (next one might one up anyway), lots and lots of concrete and steel is used for containment etc. Additionally, the whole construction process is hopelessly bureaucratic and takes forever, requiring inspections of inspections, redesigns for things that in, say, coal plants would just be redone differently by contractor on site and so on. We most definitely could make fission significantly cheaper, and in fact it has been so in the past.
Now, fusion does not have similar safety requirements. If the plant blows up for some reason, there's no significant radionuclide contamination. Therefore, there's little reason to require safety standards significantly higher than those for coal plants (the boilers of which can also catastrophically blow up). This makes it significantly cheaper, due to reduced material use and construction time/labor for containment, and reduced regulatory delays.
Your point about power density is also good, but I don't think it matters very much in practice. Sure, fusion reactor might have significantly lower power density than fission reactor, but so what? Renewable plants are even worse when it comes to power density, yet it doesn't stop them from being rolled out. We don't lack space for fusion plants, even if they are much larger than equivalent fission ones.