That doesn’t make a decision “wrong.” Lots of Supreme Court opinions are wrong, and often obviously so. There is nothing wrong with overruling them.

Citizens United was an instance of the government sliding down a slippery slope of precedent, which forced that precedent to be re-evaluated. The facts (the government attempting to punish someone for distributing a political movie about a political candidate) were so stark, they revealed the defects in the precedent that may have been obscured previously. Moreover, the government made clear at oral argument that there was no limiting principle to its position—it acknowledged that, had the Court ruled the other way, nothing stopped the government from book banning and other core free speech violations.