No worries, I didn't think you were being curt, I just wanted to be sure I was understanding what you were trying to tell me. My goal is to add light not heat to the discussion (thermo pun).

We definitely approach this sort of thing differently. To use your first example:

"So it seems almost beyond dispute that the capital cost of building a fusion plant will be immense -- far, far larger than any other type, and that would be true for any type of fuel that hasn't been used before, purely because it's Plant #1. Plant #100 always costs way less."

I completely agree that it may turn out that building fusion plants are prohibitively expensive. That isn't "disputing" the assertion it is simply accepting that it may or may not turn out to be the case.

The way I approach it however is this; Let's assume there are a bunch of possible futures. Some of them are like "Fusion plants cost 10x more than even Nuclear plants" and that future is not interesting from discussion point of view because the market won't allow that sort of thing to survive. So let's consider what would indicate that they would be financially feasible.

So looking at that one question, we have a couple of interesting examples. There is ITER which is consuming billions of dollars.

Not all of the expenses there would be present in building the next one as you note. Still, even if they were half as much the plant would be hugely expensive.

Then we have the Wendelstein 7-x stellerator[1],
the HIT-SI system[2] at UW, the Lockheed Martin entry[3], and the MIT ARC project[4].

So in one future one of these alternate implementation strategies "wins" and we get fusion plants that are both functional and affordable.

So if we're going to talk about something we can talk about all the possibilities.

So we can be in complete agreement that fusion may never be feasible while still talking about what would need to be true so that it was[5].

And I'll wrap up with this, you wrote:

"As OP said, even with free fuel, the capital cost of a geothermal or waste heat plant makes it noncompetitive with natural gas, and your argument is that fusion will produce so much electricity that the inequality will change.

So it costs huge sums to build the thing, and that can only be worthwhile if it produces huge amounts of power. More than 3 Gw. Furthermore, any downtime is horrifically expensive: you have to keep amortizing those construction costs."

That wasn't what I was trying to communicate though. IF the cost of production for a fusion plant is comparable to the cost of production of a fission plant, and the fusion plant can produce twice as much energy, then based on the LCOE of Nuclear plants the LCOE of fusion energy would be a market leader. So what I do is look for things that might inform the question, "What's it going to cost to build a fusion plant?" and try to glean any insights I can from what I find.

I also agree that if fusion plants cost much more than nuclear plants to produce the same amount or less energy, then they will not be successful in the market place.

Everyone I've talked to who is trying to build commercial fusion are targeting for much less expensive than nuclear but success is never guaranteed.

[1] https://www.ipp.mpg.de/w7x

[2] https://sites.uw.edu/hitsilab/

[3] https://lockheedmartin.com/en-us/products/compact-fusion.htm...

[4] https://library.psfc.mit.edu/catalog/reports/2010/15ja/15ja0...

[5] Yes, some people will say "Well its a waste of time to talk about something that you don't know if it can even be done." And yet, for me, it is a pragmatic use of time because unexpected things happen all the time. A case I lived through was the GM "EV-1" electric vehicle where many engineers I knew wanted to stop talking about electric cars because GM has "proven" they weren't feasible and no one would buy them because they would take to long to recharge and no one is willing to wait that long at the recharging station. That was all true during those discussions. But Tesla showed what you could do if you built one differently, and now everyone seems to think electric vehicles are the future of all cars. So for me, it isn't worthless to think about these things.