(presently apparently physically impossible)

I understood a pretty important element in the cryonics argument is assuming that you stick to things that are feasible given our current understanding of physics, though not necessarily given our current level of technology. Conflating technology and physics here will turn the arguments into hash, so it's kinda important to keep them separate. It's generally assumed that the future superintelligences will obey laws of physics that will be pretty much what we understand them to be now, although they may apply them to invent technologies we have no idea about. "Things will have to continue working with the same laws of physics they're working with now" seems different to me from "any random magical stuff can happen because Singularity", which you seem to be going for here.

I'm not sure if "just don't break the laws of physics" is strong enough though. Few people think it very feasible that there would be any way to reconstruct a human body locked in a box and burnt to ash, but go abstract enough with the physics and it's all just a bunch of particles running on neat and reversible trajectories, and maybe some sort of Laplace's demon contraption could track enough of them and trace them back far enough to get the human persona information back. (Or does this run into Heisenberg uncertainty?)

The "possible physically but not technologically" seems like a rather tricky type of reasoning. Imagine trying to explain that you should be able to build a nuclear reactor or a moon rocket to someone who has never heard of physics, in 1920 when you don't have the tech to do either yet. But it seems like the key to this argument, and I rarely see people engaging with it. The counterarguments seem to be mostly about either the technology not being there or philosophical arguments about the continuity of the self.