The 747 is probably nearing the end of it's production run after 35 years and counting, to be replaced by whatever economics dictate is the next logical step.
Airlines are still ordering 747s, though they are the 747-400F ER versions. Regardless of belly space in passenger aircraft, they will be used for a LONG time for outsize cargo. The AN-124 is a bit specialized (and probably a bear to maintain), the AN-225 is WAY too specialized and short-legged, and the A380's freighter deck configuration will not allow large height/diameter loads.
Also, as others have pointed out, the passenger version will be in service for a long time for long overwater routes.