^How exactly would Warner Brothers lose the rights to a comic book character owned by a comic book company they own ?
Under copyright law a creator or the heirs of a creator can try to reclaim all or part of the copyright over a character once a prescribed number of years has gone by from the character's first publication. Some of the Superman rights are approaching that time frame and Jerry Siegel's heirs are trying to reclaim those rights.
There are, though, a lot of erroneous assumptions and oversimplifications about the ongoing legal case and the rights situation being thrown about online. Warners can't put a stake through the heirs' claims by making another film in the next couple of years. If the heirs win their case they'll reclaim certain rights no matter whether Warners produces a film now or not. And if the heirs do regain some of the rights it won't give them complete control of the character. Warners/DC would still control many elements of Superman copyright and they'd still retain Superman as a trademark.
One of the big issues in the case is whether the heirs are entitled to be paid out a percentage of everything Warners and DC have made from Superman and, if so, how much. Obviously having that as an unresolved issue puts the financial picture of any prospective Superman film in doubt. They took a big financial hit when it turned out that legal due diligence hadn't been properly done regarding the rights to Watchmen
, so it's not at all surpising that they're holding off on a new Superman film until the legal issues are resolved.