I think you're right, and I also wonder: Considering how hard it is for us to engineer a fusion plant that merely "breaks even" in terms of energy made usable to us, will fusion plants ever be competitive with fission plants? Like, I understand that hydrogen is a lot cheaper than plutonium or enriched uranium, but iirc we never had an issue getting fission plants to break even. As far as I'm aware, the technology for exploiting fission was obvious as soon as they realized that radioactive decay could be hot enough to boil water, and at risk of sounding glib, everything else about it is just safety measures. Whereas there is no fusion equivalent of "The Pile"[1] from Fermi et al.
I do expect fusion to some day be the dominant source of energy, at least if we intend to continue to increase our energy consumption, instead of going all Amish.
But that MAY be after we spent all the easily extractable uranium and (unfortunately) fossil fuels.
When looking at costs for almost anything, there tends to be 3 main components:
1) Raw materials for construction. (Edit: + energy consumed)
2) Raw materials for fuel/operations. (Edit: + energy consumed)
3) Labor
Fusion may require very significant amount of raw materials for construction, and also (with today's technology) a lot of labor. But it is nearly free in terms of fuel costs.
Now, let's assume (as a thought experiment) that AI will make labor virtually free within 100 years, we're down to fuel costs vs the materials needed for construction.
As long as we live in a world where we expect capital to yield some kind of significant/exponential "return on investment", we can calculate a "present day value" based on what we expect that return on investment to be.
But it's not a given that the assumption of exponential growth will hold true in the future Or rather, it's almost certain that SOME DAY it will cease to hold true. In particular, when we run out of other fuels (and we've covered Earth in solar panels), stagnation is inevitable. That means the natural interest is basically 0.
In such a world, and with the assumption that labor is free, fusion would give "free" energy, since capital costs no longer matter compared to the present day value of a permanent revenue stream.
So, given enough time, it seems clear that fusion will almost certainly take over as the dominant energy source. The question is when.
If our energy needs remain moderate for quite a long time or if we get a 300 year AI winter, it could take 100s of years for fusion to become dominant.
On the other hand, if AI makes labor virtually free 50 years from now (or makes labor only slightly more expensive than the energy used), and we continue to need more and more power to drive our data centers, manufacturing and every other need we may have, we may need it sooner.
Also, if we're not able to progress beyond barely breaking even in terms of energy produced vs energy consumed in the process, that will make progress slow.
But if we continue to make progress in terms of energy efficiency, and especially if we're able to scale fusion power to generate very large outputs (10's, 100s or 100s of GW), it may take over relatively quickly.
If I were to bet, I would guess it will take somewhere between 50-300 years for it to generate more than 50% of our power requirements.
[1] https://en.wikipedia.org/wiki/Chicago_Pile-1