Ghost Protocol opened a week early in 425 theatres and made close to $13M. By the time it had that $29M gross you mention, it was already sitting at $62M. That was also in the middle of winter holidays so not really comparable. Its second wide release weekend was down 0.5% from its first.
MI3 almost killed the franchise, since it was another $150M movie and made a disappointing $134M domestic and $395M worldwide. MI2 made $545M off a smaller budget 6 years prior, and even MI1 made more money back in 1996.
I am sure that Rogue Nation will do excellent business overseas, but a $50M opening for the series is not all that great. The franchise had openings in that range 20 years ago.
Mission Impossible 2 opened to $57M over 15 years ago, and that was after the film opened on a Wednesday burning off $20M in business. This isn't a high water mark. Given typical summer legs at this point in the year, MI5 will likely ended up the second lowest grossing film in the series domestically with a $50M start.