There's no other sane way to negotiate large international treaties. The moment you open it to the public tens of thousands of political advocacy groups, lobbying groups, interest groups, protesters, etc from each country start yelling about all the things they want changed. You can't negotiate if your needs keep changing or are conflicting.

Whether we like it or not the TPP really is the "gold standard" of international treaties. Unfortunately it turns out no treaty at all is probably better anyway.