Companies will have come to base their product plans on the fact that they can incorporate patents from the various pools. It's not fair for companies to permanently be at the mercy of others who may destroy these planed products by removing patents from OPL pools on a whim, so three months after OPL version 1.0, the escape clause time will be reduced from three months to 10 days. Then the concern about whether it's a good idea for a particular company to participate or not will have changed from a concern about whether the OPL has unforeseen advantages and disadvantages, to a concern that patents not be mistakenly submitted. Those latter mistakes can presumably be found within 10 days of submitting patents, but finding out about former mistakes can take months. The license is designed to allow for enough time for both types of mistakes to be corrected, while still being fair to those who remain participants.
Patents are fundamentally monopolistic to begin with.
This is an agreement to not act monopolistic with anyone who agrees to the same thing. The only requirement to obtain the benefits of the use of the patents submitted is that you abandon your patent monopolies at least to the extent that the licensor of those patents did. My vague understanding, and I would appreciate any feedback from lawyers, or anyone who knows about the issue in more depth, is that the US Justice Department tends to take a dim view of patent cross-licensing when it is used to maintain a monopoly. The purpose of the OPL is basically to break an unfair and harmful monopoly of certain types of IP, so I don't think antitrust problems will be a large concern. But again, I would welcome more detailed info or pointers on the subject.