Billions is not an incredible amount of money.

Billions with nothing to show for is.

Fusion energy is the first seriously pursued energy source which has never been proof-of-concepted for more than a few million dollars, and its only competition there is nuclear fission. Even there I suspect that the first criticality would have been achieved at even lower cost. I'm digging for references, though AFAIK first human-triggered criticality was at the University of Chicago in 1942 under Enrico Fermi (I'm not sure if earlier radiation workers achieved criticality). I can't find budgetary amounts for that project, but the entire Manhattan project was $26 billion in 2014 dollars, and it delivered sustained chain reaction, working reactors (at the very beginning of the project), and an actual stockpile of working weapons, two of which were used in combat.

By contrast, fusion research, while providing weapons, has not succeeded in delivering sustained net-energy-positive reactions.

And every single prior external energy source humans have utilized has been proven on a budget of at most a few dollars, if not freely in nature: agriculture, wood fire, coal, oil, gas, water, wind, solar, geothermal, wave, tide.

Another distinction which occurred to me some time back is that fusion is the first (on Earth) non-chain-reaction type material-based energy source.

That is: all other energy sources involving transformation of matter (whether by conventional molecular or nuclear chemistry) will proceed by themselves under proper conditions in which energy is sufficiently reflected or retained at the point of reaction.

In the case of solid fuels, a fire is achieved by piling fuel such that airflow is permitted (providing necessary oxygen), while the heat of burning embers is contained to allow more fuel to participate in the fire. Many sufficiently volatile fuels will burn without much or any concentration (e.g., candles, matches, twigs), though a coal or log fire generally requires _some_ assembly. Those conditions are readily and directly observable in nature.

Similarly, sustained nuclear fission is accomplished by ensuring that emitted neutrons have a sufficient likelihood of striking more atoms that the reaction is sustained. This is easy enough to achieve that nuclear industry workers have to take pains to not accidentally create critical masses of fissible materials (a task at which there have been multiple failures).

Wind, water, and solar energy demonstrations and practical application all pre-dated the Industrial Revolution.

By contrast, fusion technologies are all based on the principle of feeding more energy into the reaction chamber to continue the reaction, and absent that very active feedback (through very complex means). It's only when you, very literally, achieve stellar magnitude concentrations of mass that fusion occurs naturally at rates sufficient to provide substantial amounts of energy. And even the Sun, a third of a million times as massive as Earth, only undergoes hydrogen fusion at a rate per unit mass roughly equivalent to reptile metabolism. You, as a mammal (I presume), are releasing four times more energy per unit mass than the Sun is. The Sun just happens to be slightly more massive than you (or, of course, your mother -- or mine).

While exothermic fusion has been achieved on Earth, it's not been controlled, sustained, or utilizable for productive energy use. And I really don't see that changing.

When you contrast the billions to trillions in research that would have to be spent simply researching useful fusion technologies with the amount of actual energy infrastructure which could be constructed with an equivalent expenditure based on solar, wind, geothermal, and liquid fuels synthesis (among other known and proven technologies), there's really no argument in my mind.

Keep ticking the research along at low levels, sure, but in the near term (the next decades to century or so) we really have an exigent need that should be fulfilled first.

Finally: "Population has absolutely nothing to do with carbon output levels" is flat-out false, and misses the point. The accepted formula is in fact dependent on population, but also affluence and technology: I = P × A × T

That is: impact (including CO2 emissions) is a function of population, affluence (net resource consumption), and technology. https://en.wikipedia.org/wiki/I_%3D_PAT

The other problem is that it's not just CO2 emissions but total carrying capacity which are limited, as exemplified by "peak everything": oil, water, topsoil, phosphorus, uranium, copper, and many other inputs to both industrial and nonindistrial human existence. Estimates of what specifically a long-term sustainable carrying capacity might be vary, but a number that has been suggested (by the Ehrlichs among others) is around 2 billion. Last I checked we'd exceeded that slightly, and there might be a recruitment of volunteers in the near future.