With enough surplus energy, you could run entire reactors just for carbon sequestration, or nation-scale desalination, or climate engineering, or what have you.
In a simpler way of looking at it, (1) what is the source of the energy of solar/wind? (2) how much land and materials are required to linearly increase power production?
There is a finite amount of space and resources on the planet to continue to scale power production with humanities consumption.
Fusion, preferably MCF/tokamaks in the style of smaller sized ones like SPARC @ MIT and less like ITER (behemoths that take decades to build and maintain) offer two things (1) the fuel is comprised of the most common elements in the universe, (2) power per square foot is much greater than solar or wind... And bonus (3) once developed it should in theory require less material per watt generated. And less materials mean less processing and fabrication which in turn reduces the environmental impact on the planet.
You seem to want to halt all further research into alternative energy and settle with the current state of our solar/wind capabilities, which is strange.
We can do both.
Seasonal variation with solar is a bit of a bummer. If we need to fully electrify everything, (Transport and heating) then winter will be a problem. Either we massively overprovision solar in order to still have heat on the shortest day of the year, or we run thousand mile cross-country transmission lines and enormous battery banks.
Even so, the economics are such that heavy industry might become a seasonal job. Right now we run aluminum smelters 24/7 because baseload power is fairly consistent, but if solar power is free in July but dear in January you might see multi-month shutdowns. This gives headaches to central planners, and makes them inclined to pour billions into fusion if it can preserve some of the status quo.
Or it will move to locations where seasonality is not as important.
Many equatorial regions will either be too hot for human life, (https://advances.sciencemag.org/content/6/19/eaaw1838) or be active combat zones as a result of refugees escaping heat. Some cities in India are now routinely hitting 50C during the summer, and we've only had 1C of warming.
If fusion works out, it could be used to make up the difference when the sun isn't shining and the wind isn't blowing (though if we eventually get high-capacity transcontinental HVDC lines to buy and sell power from practically anywhere or batteries become really cheap, that becomes less of a concern).
Fusion would also would require far less land, and some people object to having a landscape covered in windmills and solar panels.
Fusion might be useful in places where renewables are less practical, like on ships. Naval vessels might conceivably replace fission reactors with fusion. If they're safe and relatively simple to run, you might even see them on civilian ships. Or you could have fusion reactors in remote places, like floating on a buoy in the middle of the ocean, to serve as a charging station for battery-powered ships.
Fusion may be useful for establishing a human foothold outside of Earth. For instance, methane production on Mars (for rocket fuel) will require enormous amounts of energy, which could be supplied by a fusion reactor. (A fission reactor would perhaps work just as well, but there are legitimate reasons why people get nervous about launching hazardous materials into space on a rocket that might blow up before it achieves escape velocity.)
We might also begin engaging in projects that require enormous amounts of energy. For instance, if certain CO2 absorption strategies are energy-intensive, and we can't practically generate that amount of energy from renewables.
At this point, we really don't know if it'll work much less what the practical limitations will be, so perhaps the best we can do is say "if a fusion reactor can produce X amount of energy and weighs Y tons and requires such-and-such amount of cooling and requires an overhaul once every N months at a cost of D dollars, we might want to use it in these applications".
Fusion would be horrible for ships. Ships are volume constrained, and fusion reactors are very large.
Land constraints are not globally significant at current energy demand. The world is constantly hit by 100,000 TW of sunlight; average global primary energy demand is about 18 TW.
In space, DT fusion reactors will be inferior to fission reactors, which will be much smaller and lighter for a given power output (and also much simpler).
It's very difficult now to make a case for fusion. In the past, the case was something like "fission will be a big winner, but then we'll have trouble with uranium availability and safety and waste, and fusion, while slightly more expensive than fission, will still be cheap and solve these problems." But that's not how it turned out -- fission failed because it was too expensive, and fusion being even more expensive than fission makes it a nonstarter.
Currently, we don't have any practical working fusion reactors, so it's hard to say what the attributes of such a reactor would be. We have some designs that according to our understanding of physics might work, but the designs are likely to go through many iterations before we have something that can be mass-produced and deployed in volume. Rebco tape probably isn't the best high-temperature superconductor that will ever be discovered. And so on.
It does, actually, with neutron producing fuels. The problem is that volumetric power density is limited by the areal power density limit on the wall of the reactor, and by the need of a sufficiently thick blanket to absorb neutrons. The inferiority vs. fission is roughly (thickness of fusion reactor blanket)/(diameter of fission reactor fuel rod). This is independent of any details of plasma confinement.
Something like ARC has much higher power density than ITER, but it's still very inferior to fission reactor. ITER's power density is just so incredibly bad.
There might be limits though on how small the reactor can be made. ARC is apparently meant to produce hundreds of megawatts, which sounds like it maybe be two or three orders of magnitude more powerful than what even a large container ship would use for propulsion. SPARC is a physically smaller reactor, but not intended for continuous or long-term use. If the basic design works out, probably the first real-world designs will be optimized for utility power generation, where size doesn't really matter except to the extent that "bigger" tends to mean "more expensive". Minimum-size designs might take longer to show up.
If I understand correctly, the ARC design uses FLiBe to capture the energy from the neutrons. It takes up the space between the vacuum chamber and the outer housing. The FliBe heats up, and is pumped out into heat exchangers that produce steam to run a turbine. At some point there's a practical limit to the amount of heat that can be removed that way, but it seems like a low-output reactor should be easier rather than harder to make from that standpoint.
This is an ~80 megawatt ship engine. https://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA9...
"Hundreds of megawatts" may be oversized for ship application, but maybe more like 2x to 10x oversized rather than 100x to 1000x. Or maybe not. Apparently they use some pretty powerful reactors in aircraft carriers: https://en.wikipedia.org/wiki/A4W_reactor
So the footprint of fusion would be a lot smaller.
Also for the same reason deployment would be faster allowing a faster phase out of fossil fuels.
Deploying solar and wind does not preclude continued research and development into alternative energy production methods.
Fossil fuels had better be phased out soon, and fusion cannot be available in that time.
IMO, the prospects of fusion reaching a practical state are so remote that even the current level of funding on it is difficult to justify. There are fundamental engineering constraints that render it inferior to fission -- and fission is now going extinct itself, being too expensive.
Why are you excluding renewables? Argue quantitatively and show your work.