CHEERS

Firstly, I'm a big Sam-Diane fan - for what it was & the time they had, not for what ifs or hypotheticals.  They had classic stage worthy scenes & moments together that remain peerless.

I used to feel like her leaving cheated the show's complete legacy.  Theirs was found gold for tv relationships of all time.  Hindsight I now seem to think that Sam & Diane both were simply incapable of lasting together.  Recall, S5 was them accepting marriage, yet with having to propose the 'right' way - they were fighting over how to even begin marriage off the right way.  Like polar opposite magnets if you will - they both wanted one another & yet they both couldn't have one another the way they wanted.

I don't fully fault S.Long for leaving the show as much as I used to either.  She had a 5yr contract & merely opted not to re-up once Disney already was paying her more; + she wanted to spend more time with her family & not the sitcom grind.  It wasn't like she was demanding more money or holding the show hostage.  Like I said earlier, I think Diane-Sam had run its course to its nth degree.

I've read they actually filmed 3 different S5 final scenes - a. her book deal (leaving Cheers), b. wedding/marriage (staying), & c. one the producers didn't reveal.  FOX aired the wedding as a deleted scene & can be found online.

Had she stayed, I'm not certain how Cheers might've steamed ahead to the 90's.  The writers dialed Sam (& subsequently the entire show's dna) towards more silliness & slapstick humour (like his dealings with Rebecca, John, etc), most certainly not jiving much with a Diane-Sam marriage.  
The rest of the ensemble seemed to breathe easier without their focal tension, surely giving the show longer legs to thrive.  Remember, Diane & Sam were no longer youngsters on the prowl - they would've been a maturing couple.  

I always thought they at least might've reached out & had Long do a couple recurring cameos - it was as if she'd fallen off the face of the earth which for all she & the show put in for the character seemed cold.  

The Diane-Sam senior flashforward scene they did air remains canon for me - though I've always instead imagined them more as cohabitors than lovers.  Maybe when they hit 60 & sick of working &/or dating, they just find a house to move into together & ride life out together, unmarried.  
Her goodbye with Sam's 'Have a good life.' remains one of the heaviest emotional spots in tv history for me, & yet as I've said there was really no place else left to go for them both on the show.  

A blessing in disguise perhaps - the world still discusses all of this with the show.  Unlike say Friends where the Ross-Rachel nonsense was watered down, blown up, etc etc until they landed with the safe ending - which was barely memorable.  
Diane-Sam eat Ross-Rachel for breakfast & twice on Sunday.

Side note- one reason I prefer S4 as my fav is (besides Woody's intro whom I love) S1-3 still felt late 70's era which was before my time, & S5 while pretty epic felt more constrained with the weight of Diane-Sam marriage drama.  Them both single in S4 let the comedy shine through brightest for me.