First, no one would have to dual stack. Same stack, larger address space. Routers would do wire format conversion where necessary. Second, technical errors in the TP/IX RFC have no bearing on the merit of the principle described. The IETF was just incredibly short sighted on that point, preferring a start over from scratch design to one where the transition would be practically seamless.
In short, the counterargument here has to be not against the weaknesses of a seventeen year old network proposal per se, but rather against the entire idea of preserving the existing address space on a long term basis without renumbering.