The ending settles the supernatural fight and the legal one in different ways: Ed and Lorraine's bond breaks the curse, but the court still convicts Arne. Spoilers follow.
How does the curse end? Lorraine's love pulls Ed out of the curse, and he destroys the occultist's altar. That frees Arne, and Isla dies from the evil she summoned. The film treats the Warrens' marriage as a weapon, and it could be argued that the whole saga leans on that idea. It is also a callback with a twist. In The Conjuring 2, Lorraine's séance at the Amityville house gives her a vision of Ed being impaled, so the series has been dangling Ed's mortality in front of us for a while. Here his heart attack in the opening exorcism, which the filmmakers say is based on something that happened in real life, finally makes the threat physical, and Lorraine is the one who has to hold the line.
Why does Arne go to prison anyway? The court convicts him of manslaughter, he serves about five years, and Debbie marries him in prison. In my opinion this is the most interesting choice in the script, because the film does not give the Warrens a legal victory in the way a courtroom thriller usually would. The demon is defeated, but the sentence stands. The film sets up the Warrens' defense as a first in an American murder trial, so the verdict reads as a refusal to let the supernatural settle a human case. Compare that with The Conjuring (2013), where the Perron family's problem ends with the haunting. Here the haunting ends and the consequences remain.
What is the cup in the final shot? Ed places a cup from the altar in the Warrens' artifact room, next to cursed items from the earlier films, such as the Annabelle doll. That room is the connective tissue of the saga. In Annabelle Comes Home the Warrens lock the doll in a blessed glass case in the artifacts room at their home and leave Judy and her babysitter there, so this closing image works as a quiet nod to the spin-offs. It also points forward, since Last Rites later brings back an antique mirror as a cursed object in the Warrens' world. For the doll itself, see our Annabelle doll guide.
Does the ending hold up? Perhaps the weakest part is Isla herself, a villain who arrives late and dies quickly, compared with the long build of Valak in The Conjuring 2. The strongest part is the human cost left on screen. If you enjoy the series for its scares alone, the finale may feel quieter than the first two, and that is a fair complaint.