The consensus among Cheers aficionados, TV critics, and anyone with a brain consider Chambers vs Malone the worst episode in the show's history. Diane had already become insufferable by this point and her welcome had worn out well before this season even began. Here we have her suing Sam to get him to propose and a judge actually forces him to do so. The laughs are few and far between. Diane is just an annoying idiot who is so desperate to get what she wants that she will stop at nothing. That's the key here. She doesn't even necessarily want SAM. She just wants him to give her what she wants: a proposal and eventual marriage. If she were truly in love with him and wanted what's best for him, she would never ever behave the way she did here. However, she realizes she's pushing 40 and has no marriage prospects, her chances at motherhood are close to nil, and her life is pretty much a failure with regard to career opportunities. Therefore, she is trying to turn back the clock and fix some of the mistakes she made. Hence her forcing Sam to propose.
Can you imagine if the wedding had taken place? As Sam realizes later on, they would never have been happy together and it's largely Diane's fault. She could never be happy with Sam, nor could she be happy with someone similar to her (like Frasier). She could never be happy with anyone because she is an insecure, anxiety-ridden person who needs to constantly talk, lecture, and argue to feel like a whole person.
By this point in the series, the writers had no idea what to do with Sam and Diane. They were afraid to abandon the relationship because it was part of the reason the show had such great ratings. They were afraid that without the conflict of the relationship, the show would fail. Boy were they wrong. Cheers saw its highest ratings ever in the Rebecca years. At this point, the jokes were stale, Diane was more annoying than funny, and the audience seemed tired of the same old back and forth between them. Even Shelley Long had grown sick of playing the same old storylines over and over which is part of the reason she left the show. She felt they had said all they could say about her character.
I remember this season when it aired and used to wonder what the big deal was with Sam and Diane. It seemed pretty clear nothing was going to happen and neither Sam nor Diane were my favorite characters to begin with. I always wanted more of Frasier and Norm who were the real comedic geniuses of the show in my opinion. Luckily, Diane's departure allowed for the other characters to grow because there was a lot more room for other stories.