Honestly guys, they'll just have to worry about sequels if AND when they happen.
We've been waiting a long time for a decent Superman movie. There has been numerous false starts, some extremely poor scripts/storylines which never made it to fruition and so on. We got Superman Returns in 2006 which had a very lukewarm response, and ever since then fans have been clamouring for all the things which that film lacked - a better villain, more action, a better suit, more colour, stronger storyline, etc.
We're finally getting what we asked for. A near perfect actor for the Superman role, a great supporting cast, tons of action, moving away from Donner's films, and much more. And if they went absolutely all out on this film and gave 110%? Good for them. How they'll top it in a sequel a few years down the line is their problem, and that's why they get paid a lot of money to come up with ideas which work. Have faith