I guess Ian is one of those tech fetishists who think if it has the same graphics it's the same console even though it had massive developments in the user interface sector. Some other important people said they consider the Wii the only real next gen console because the others are just the same old with more graphics while the Wii provides a real step ahead that the user will actually benefit from.
Interface improvements aside, the Wii is more powerful than the GC. Isn't it supposed to be as powerful as 2.5 gamecubes, or something like that? Obviously, its not as much of a leap forward as the 360 or PS3 was over their predecessors, but it is an improvement nonetheless. Remember, the Wii and the GC share the same chipset and architecture, but Wii games would NOT run on the GC, because the Wii hardware is more powerful.
I suppose you could look at it like how an IBM compatible PC from a few years ago seems so similar to an IBM compatible PC from the current year. Sure, an older PC and a newer PC will be about the same to program for, and odds are 99% of new software will run on the slightly older PC as well, but that's not to say the newer one isn't faster and more powerful. So again, there IS an improvement, but understandably it might not be as much of an improvement as Ian and most gamers would like....
And even if graphics were EXACTLY the same, that would completely ignore the countless other enhancements the console introduced; including everything from the controller to the virtual console to the internal memory to the WiFi support. All of these and all the other enhancements are enough in my opinion to consider the Wii a successor to the GC, even if graphics are only incrementally improved.