But I submit that the more powerful the 3rd party is, and thus, the more they are able to act as a good contract enforcer, the more they actually assume the power and role of a government.
Well, you've basically hit the nail on the head in identifying the trade-off that is at stake here.
You are correct that a government is better able to enforce contracts, laws, etc. than a private entity would be. However, government is also better able to violate your rights, steal your property, murder you, etc. than a private entity would be. The voluntarist argument, as I understand it, is not that government is completely without merit, just that the excesses and abuses outweigh the benefits of "being more affective at enforcing contracts."
And this particular trade-off doesn't just show up in a "government vs anarchy" debate. It also shows up in any argument about what kind of government to have. Remember, back in the day, people essentially made the same argument you are making to justify monarchy. We need to have a King, because they're so much more capable of really "getting things done", right?
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