When Gulfstream introduced the G500 and G600 in October 2014, they said that the new aircraft were not meant to replace the G450 and G550. Instead they are the beginning of a new product family, and the market would decide if the G450 and G550 were still needed.
The announcement that the G450 would end production came in October 2016. By the time the last aircraft is delivered, Gulfstream will have built 365 G350 / G450s.
The G550 announcement might take a bit longer. Gulfstream has just delivered the 550th G550.
Deliveries of new aircraft have slowed. Gulfstream said in May 2015 that they had delivered the 500th aircraft.
Gulfstream has not delivered this many aircraft of a type before. The Gulfstream IV family did get very close, but including the G300 and G400 they built around 535 aircraft.
You could argue that the G350 and G450 should be counted in the Gulfstream IV family, which would take the total over 550. But if you did that, you would have to include the Gulfstream V in the G550 total as well.
Many good aircraft do not get this far. But Gulfstream do not make good aircraft, they make great aircraft.
Eventually the G600 will replace the G550. But with 50 deliveries over two years, Gulfstream will be in no hurry to stop building G550s.