I see an awful lot of successful creatures around with 5 fingers myself. Although success can be measured in many different ways. Beetles are incredible successful if you count success in terms of number of species, and I doubt they have 5 fingers. Where you mention that..

Quote - The other lineages died out, and the one that was left just happened to have five digits on each limb.

This to me contains basically the same information. There were creatures with different numbers of fingers. Some of them died out and some of them didn't. The 5-fingered line continued. Personally, from my own POV, I'd count that as successful compared to the lineages that died out ;) I think it's remarkable the degree to which the five-fingered clan thrived compared to its close relations. so I think, on the whole, that five-fingered beasts HAVE been remarkably successful. Cheers, Cliff