Yes, I know the stories about the fact that it is extinct. I wonder though how many people have been to all the various islands in the Isla de Cedros and Guadalupe region?
I agree with you that there are a number of plants that we know as good species that have forms with straight spines and forms with hooked spines. As well as capensis, there is albicans, marcosii, rekoi, just to cover a few Series! We also know that the genetic profile of plants can produce varied forms from a single plant. They seem to contain a strong propensity to evolve. I've even seen a red flowered dioica.
However, another reason could be that two close species have interbred, and given rise to a very variable mix of plants. Sometimes the original parents might well have died out, and so what we see as a result is a variable species. Or perhaps they do still exist, in the same locality and then what we see gets even more complicated. As far as goodridgei, I have read (but can't locate the reference at the moment, unfortunately) that the straight spined form grows mainly on one side of the Isla de Cedros, and the hooked spine on the other, with mixed populations in between. Now that could just be geography favouring one form, or it could be something else.
What I'm really saying is just don't rule out some of the other possibilities - botany is an approximate science, and judgements are made often without as much data as the botanists might like. As one who trained as a physical scientist, I would love clear rules, that x follows because of y, but the plant world isn't like this. Sadly!