It's amazing how long it takes these popular memes to catch up with with scientific progress.
The Dyson sphere and the Kardashev scale were brilliant ideas in their time, but today we know they are the wrong approach. Collecting solar energy in this way would be incredibly inefficient and expensive.
Once fusion becomes functional, it makes far more sense to use the sun as a fuel source and use a bit of the hydrogen to generate needed power.
In the grand spirit of futurism, the engineering challenges of such a system are left as an exercise for the reader.
A Dyson Swarm is far more mass efficient than building fusion reactors to generate equivalent power. Remember that a dyson swarm is simply the radiator for a fusion reactor.
Further, the sun is not a fuel source for artificial fusion reactors. The hydrogen that makes up the sun is the wrong type. Fusion fuel is abundant by kardashev I standards, but it's not even close to being enough for a kardashev II civilization.
Hell artificial fusion isn't even very competitive with fission besides PR.
Fusion reactors can breed their own tritium. In fact, they will have to do so if fusion energy is to become widespread. Furthermore, pound-per-pound of equipment, a fusion reactor should generate more output than solar PV.
The larger point is how these ideas get lodged in the culture. Looking for stelar anomalies in line with a Dyson sphere/cluster is an accepted part of the search for extraterrestrial life.
The speculation about the future from 60 years ago needs to be updated occasionally.
Tritium isn't the concern. Fusion reactors consume deuterium and lithium. The sun doesn't have much of either. If you consumed all the Deuterium and Tritium in the solar system, you'd only get the equivalent of a few thousand years worth of the sun's power output. You also need various exotic materials for the reactor construction. Just matching earth's current power generation requirements, for example, would probably take more beryllium than exists on Earth.
Fusion reactors do not produce more power pound for pound than photovoltaics. ITER, for example, will produce 21 watts per kilogram, space based solar panels produce about 200 watts per kg. And again, if you were to place a fusion reactor in space, you also need the radiator panels as well.
Looking for dyson swarms is an accepted part of the search because it's something we expect to find. We knew about fusion long before the dyson sphere was posited, and everything we've learned in the intervening decades has made fusion reactors look less appealing.
You don't want to make too many assumptions about where a civilization that can construct things mesuring several AU and harness yotawatts of power find habitable.
My though was that if they can create a swarm like that, they can also partially shade the earth to cool it. This would only work in the partial scenario which has 3 degrees C warming. 140 degrees for capturing all of the suns output is huge.
