The redefined environmental standards are officially the main reason for the latest renovations. The brand new claddings and interior fittings would be significantly more energy efficient than older ones. So although some older buildings yet insteresting should be preserved, businesses generally don't want to stay in obsolete towers. They want the latest thing to cut their energy bills and say - see how green we are! the building we live in fulfills the commitment to save Earth.
Besides, the district wasn't aging well as too many buildings of the 60s-70s in Paris look bad nowadays. So all operations going on are to bring more modernity and dynamism. It's either being on the move or declining, an easy choice to make.