When Sims 3 came out, they stopped making new EPs for Sims 2, so I'm assuming it will be the same this time. But Sims 3 will continue to be available, and certainly people can continue playing it as long as they like. Nobody will have to give up their Sims 3 if they don't want to!
Also, this forum will continue to offer support for Sims 3, and we will also be opening a new Sims 4 section to the forum.