If you look at medicine over the years, it has strongly tended to be able to cure things it used to not be able to cure. For a long time, we couldn't treat smallpox, and then we could, and now nobody suffers from smallpox. "Future technology!" invokes this trend and calling it a stopsign doesn't explain why this trend doesn't apply to cryonics.
We don't need to explain why this trend doesn't apply to cryonics. The complaint is not with the trend, it is with using "future technology" as an answer to specific problems we do not know how to solve. Its not an answer at all, its like saying "we will solve it by solving it...later".
Luke Parrish points me to what is clearly by far the most serious critique of cryonics ever written: a 57-page treatment by Evelina Martinenaite and Juliette Tavenier, presented as a 3rd semester project at Roskilde University in Denmark supervised by Ole Andersen.
Full paper