Anything is possible: The Singularitarian’s trump card

In response to the previous post here, asking what humanity might be like today if transhumanists had remade man in the 1950s, Michael Anissimov asks, “if we modified ourselves into this based on the ideology of the 50s, couldn’t we just then change it again if we didn’t like it?” This comment merits some attention because it exhibits one of the most common transhumanist tropes — a supposed discussion-ender.
Sure, one can claim that all such morphological decisions will eventually be completely reversible. One can claim that we will be able to change our forms just as easily as flipping a light switch. One can claim that people will be able to make choices without the slightest effect on other people, and that each generation can make choices that don’t impinge on the next.
But what reason is there to believe any of these things are possible? And even if they were possible, what are we to do in the meantime with a world in which they are not? And more to the point, why bother discussing futurism at all if we can supposedly do anything we want without any necessary consequences or limitations?
A defining feature of Singularitarianism is its basis in a fantasy world in which anything is possible (or at least, in which we have no way knowing for sure what isn’t). This gives Singularitarians a way of wriggling out of any argument by saying that no matter what the potential problem, we’ll be able to find a way around it (or at least, we don’t know for sure that we won’t).
I’m not sure if this is an argument from eventual omniscience/omnipotence that is tantamount to an argument from present infallibility, or if it is just an argument from the impossibility of proving a universal negative. One way or another, this is something to the effect of: Hey, why not jump off this cliff? I can’t see the bottom, but it sure looks great, and if we see any problems we can course-correct in mid-air. Which doesn’t make for great conversation.