That argument only makes sense if your utility function were either monotonic and non-decreasing over arms or if it were monotonic and non-decreasing over a com

That argument only makes sense if your utility function were either monotonic and non-decreasing over arms or if it were monotonic and non-decreasing over a combination of arms and other things (e.g. legs?). Are you sure you'd always prefer having more arms? I mean, I don't know about you, but I think most people would stop short of becoming a hecatoncheir. At best those extra arms would be useless, but in the worst case scenario you could be punished by the gods for your hubris. Either that, or be banished to Tartarus by Zeus.