It's funny how some people think they are entitled to get information from theme parks just because they do. Announcements come as they come, 15 year in advance or months before the opening. It's not up to you.
Disney is closing stuff all the time too sometimes replacing it with almost nothing (Innoventions, the wonders of live pavilion, the Odyssey restaurant, Rocket Rods, Skyway MK, 20K MK, AK+MK+DLP boat rides, Discovery Island, River country, Osborn Lights, Lights of Winter, parades etc).
At Universal there are always plans to replace attractions, there are also good reasons to close something (goes for any theme park). In the case of 8th Voyage it's probably because it didn't pull enough people in, maybe they need the stunt people for the new T2 replacement show. In the end the show isn't missed much and you would be excused to miss it when it was open because it's not in the middle of the land but tucked in the back. If you don't like it that's okay but that attitude you should get checked. it's not healthy.