ITER has horrific power density, some 400x worse than the reactor vessel in a fission power plant. Given that the non-nuclear part of the a fission and DT fusion plant will be similar, making the reactor itself much larger and much more complex, and therefore much more expensive, cannot be a win. The less experimental follow-ons to ITER (PROTO, DEMO) are still massively too large.
This generic problem with DT fusion reactors has been known since at least the 1980s.
The UK's STEP project uses a higher aspect ratio ('spherical tokamak') to reduce the required size of the machine.
The initial investigations from MAST are encouraging, but we'll see how it goes.
Of course, this stuff will be ready for commercial use in like 2050 assuming everything goes to plan (which is rare unless it's really prioritised like the Manhattan Project, Apollo Missions etc.)
So regardless, something will have to be done prior to then - either we find better energy storage solutions for renewables or we expedite the construction of replacement/new fission plants or both.
Whenever you see happy optimistic talk about a DT fusion reactor, ask them "what's the power density of your reactor?" And make sure they're using the volume of the reactor itself, not just the plasma.
For a PWR, this is about 20 MW/m^3 . For ITER, it's 0.05 MW/m^3 and for ARC (2014 paper) it's around 0.5 MW/m^3 .
ITER has horrific power density, some 400x worse than the reactor vessel in a fission power plant. Given that the non-nuclear part of the a fission and DT fusion plant will be similar, making the reactor itself much larger and much more complex, and therefore much more expensive, cannot be a win. The less experimental follow-ons to ITER (PROTO, DEMO) are still massively too large.
This generic problem with DT fusion reactors has been known since at least the 1980s.