- It is not viable yet, and probably won't be for decades, if at all
- Funding necessary to explore fusion energy's viability is at least order of magnitude $10^10
- The 'steady-state' economics of fusion energy, should it become viable, cannot presently be estimated with any real fidelity
Not only will each commercial reactor, should anybody attempt one, be extremely complex and therefore non-robust, but there's still the issue of neutron contamination, making the whole thing brittle and ready for the nuclear waste disposal team to rip out most of it in a few years time.
Production systems would likely have much higher gas contamination for example and operate at a steady state for months.
Also, ITER is actually rather small from a power production standpoint, 500MW thermal power ~= 150MW of electricity where we have plenty of fusion reactors several times that size. Luckily things get much easier when you scale fusion power.
As you scale up the power of a fusion reactor, the volumetric power density goes DOWN. This is because it becomes limited by the power/area through the first wall. Square/cube law in action.
ITER has full remote handling as it's expected to get extremely radioactive. Scaling reactors up does not really mean increasing the neutron load on the walls as you want to keep that fairly steady per surface area.
Tritium production has ~zero impact on operation. The breeding blankets are really simple, you take lithium and enclose it in a metal of some type. Replace after a few months.
Also, a ~10x device is unlikely to be 50-50 DT as while hard to operate on pure DD fusion it's not that hard to get when you have some tritium in the mix.
It's not designed to replace everything, but I doubt commercial designs would go that far either.
They've already tested joints in the REBCO tape, and found that they introduce very little electrical resistance.
These failings have been known for 35 years or more, and yet still they're largely ignored. If there is a thing called "pathological technology" (in analogy with pathological science) then fusion must qualify.
That's only 10 billion dollars. Far more than that has already been spent on fusion.