I hope it is great and the best comic book film ever made but I don't think it needs to reach that for a sequel or JLA to happen. It just has to be a big hit in the box office, doesn't need to beat Avengers or TDK either but if it can make $200mil domestically and maybe $400mil-$500mil internationally for a total of $600-$700 mil and it would be considered a success and all other projects will be green lit.
In comparison, IM series averages in the $300mil range domestically, IM1 made $320mil domestic and $266mil international. Trend these days seems to be more success in the international market in comparison to when IM1 released so MoS has to come close to doubling the domestic number and they'd be good.
At a budget of $175mil plus maybe another $20-$30mil for marketing (complete guess on marketing number), at $500mil-$600mil, I think that will put the film in the success column. In comparison, GL made $220mil worldwide with a budget of $200mil and Superman Returns cost $270mil and only made $390mil worlwide. At this point with this much hype and great early reviews, it's hard to see MoS making less than those two movies.