With a good draw and some results going their way those teams could plausibly win. Ireland are current grandslam winners and also can beat Australia (are favorites to on saturday) and SA on their day. Argentina came third last world cup and consistently beat France, sometimes England.
Hypothetical situation; ABs choke (as usual), and SA (as in 2003 WC) Australia (atm) are not in good form it is up to those other teams to determine a winner and all beat each other on regular occasions...
It doesn't matter though whether its the 3rd 4th or 5th biggest and how many teams can win. RWC Final would be bigger based on the spectator/broadcasting numbers and interest levels in NZ.