I mean... you've got to have faith in your theory before testing it out. I am not having any opinion here about this, but the cycle in my mind is theory, belief, test, updated model of reality. I can imagine similar things said before we managed to have powered flight, "Pshhhsst! We'll never fly! The law of gravity forbids it."

You do need to have unreasonable goals for things once in a while. In this case, I don't have a particularly fixed stance about SMR's, but the claim that "Thermodynamics are the reason why SMR aren't, and will never, be economical" feels a bit stronger than it is warranted. Never is a damn' looooong time. And I can easily imagine things being less efficient, but having other advantages that make them more economical. Claiming it's impossible is just stretching.