This is a far stronger argument against JS-driven websites than the environmental argument.
Would you ever make the argument: "this website would be so much easier to use with a little Javascript, but Javascript is bad for the environment, so it should remain without"?
If not, the environment isn't actually a factor in the reasoning process. It's just a club to wield when it supports what you've already decided is the correct answer.
I disagree. The environmental argument is also solid one all other things being equal. It's not just a "club to wield when it supports what you've already decided is the correct answer". Even if JS only website were okay (which they're not) and as just good as plain HTML ones, the fact that the latter is better for the environment totally counts as an argument. And at least for me, trading a 10kB web page against a 1MB JS is not okay if the JS does not bring a solid positive difference.
Now I agree that in the case we're discussing, the JS actually brings a negative difference as it doesn't enhance anything and makes the website unusable without JS and thus a lot less accessible. So here the environmental factor is secondary.
I agree that the environment could in principle be a tie-breaker. But it's such a weak argument that in practice, whenever all other things are not equal, the environment is not going to tip the scales from one side to the other. And all other things are not equal, because the environmental effects don't happen in a vacuum, they're correlated with all sorts of other things. So anyone bringing it up is just using it to support the answer they've already decided on.
Suppose the environmental impacts of the HTML-only page and the JS-driven page were flipped, but the user experience remained exactly the same - the JS-driven page still takes longer to download, uses more memory, and doesn't work unless JS is enabled; but it's marginally better for the environment, by the same amount that the HTML-only page is better in the real world. Do you now think the JS-driven page is preferable?
If not, you don't get to use the environment to argue for the HTML-only page. It doesn't sway you, so you shouldn't use it to sway others.
About your first §: Your way of thinking is worrying to me. Global warming is no joke.
About your second §: your example makes absolutely no sense, and additionally it's not at all an "all other thing being equal" case. But if I had to attempt to answer you anyway, I'd say that in that case the fact that the better environmental impact of the JS-driven version is an argument that it has for it, and at least lessen the difference in the overall evaluations of the two options.
I'm not treating global warming as a joke. I'm treating the environmental impact of javascript-driven webpages as not worth taking seriously.
If you want to take it seriously, I recommend quantifying the impact as a first step. If your calculations show it's worth taking seriously, I'll happily change my mind.
"lessen the difference in the overall evaluations of the two options" is not enough. "This is a serious factor that I care about and you should care about it too, but if it was completely reversed, that wouldn't actually change the outcome": I'm not buying it.
