This is an answer though. The vast majority of people who need address space in North America are ARIN members. These ARIN members are happy with the current organisation. If the set of people who need IP address tend towards being happy with the current system, there is no reason to change it for a new system, which they may not be happy with.
Actually, I don't believe that is completely true. The vast majority of address space in North America is given to ARIN members. However, the vast majority of people who need address space in North America are end users, most of whom get their address space from ARIN members or descendent LIRs from ARIN members. In some cases, they are end users who get address space from ARIN but are not ARIN members. Some end users are ARIN members, but, I do not believe the majority of them are. I'm not saying there is anything wrong with it being this way, just that it is an important distinction in address consumption vs. membership. Owen