IN MAY 2015 Bill Ackman, a hedge-fund manager, spoke at the Ira Sohn conference, a charity do at which Wall Street investors bat around ideas. Mr Ackman tried to rehabilitate an old concept, that of “platform” companies, which grow by continually buying others, finding their targets in the unloved and musty corners of the economy. Roll-ups, as they used to be known, were all the rage in the mid-1990s, before a series of blow-ups brought them into disrepute. Mr Ackman highlighted two firms that he said were modern masters of the strategy. Their fortunes have diverged widely since his speech,...