But it's totally an ending. It ends by opening up a whole new can of worms for the characters to deal with, but it is an ending. Several arcs reach a conclusion, and plenty of others leave off at an interesting new place.
1. Lando feels remorse for his betrayal.
2. Leia expresses her love for Han.
3. Han once again risks his neck to help his friends.
4. Luke arrogantly faces Vader and pays the price.
5. The most important one: the Empire's trap works.
Now, being the middle movie, these aren't all finite ends to arcs, but they are ends. Things at the end of Empire are drastically different from at the beginning. The main arc that ends is the Empire striking back. They set a trap and it goes off with nary a hitch, and we see the immediate fallout from it.