My feeling on it was Mycroft had to do something after Sherlock killed Magnusson in front of so many witnesses. If there hadn't been that many, he probably could have covered it up easily. Edit: Regarding when series 4 comes out, there was an article today where Moffat was quoted saying: "If people have to wait two years, they’ll have to wait two years." and "We'll do what we can, but with no sacrifice in quality – that’s the thing,” he said. “It’s not the kind of show that turns up all the time, it's just not. But the good news is, that probably means it will turn up sporadically for a very, very long while." I can't link the IGN article I originally saw it on because my work computer won't let me, but HERE is another article with it.