I think spin-offs are better than sequels in many cases. I agree wholeheartedly that stories need to fucking end and not be forced to continue on forever and just get more contrived as time goes on. I'm looking at you Resident Evil. 7 did sort of a soft reboot on the story, but Chris still punched a boulder in a volcano and whatever the fuck happened in 6 still happened in the canon.
Hmm, let's keep using RE as an example. Outbreak 1&2 were excellent spin-offs. Seeing the Raccoon City outbreak through the eyes of a bunch of random people was a great idea! Then there's Operation Raccoon City and Umbrella Corps. Oh no, chat. ORC's story was just a mess that added nothing but bullshit and UC... did it even have a real story? Looking it up I guess there was something but it was a whole lotta nothing. x) Then there's canon spin-offs like the Resident Evil: Revelations games, or I suppose they can just be called side stories, which I think is what some of us actually mean when we say spin-offs. The plots were kinda dumb and convoluted, but that's a complaint that you might as well just gloss over with the whole series because it goes without saying. Still, they were rather good, we need more like that.
Aight, still looking at you Resident Evil. Prequels. I know a couple people above said Zero was a great prequel and that's awesome, I'm glad you guys enjoyed it, I'm just going to have to respectfully disagree. Marcus was a terrible villain that feels like he should have been in an anime, not a RE game. Wesker and Birkin just kinda comment about stuff MST3K style until they're given random things to do. And finally Rebecca and Billy just kinda go places, solve the Marcus problem and then bye bye Billy! Forever, apparently. Then Rebecca and the other surviving Bravo Team members go to the mansion and promptly forget every single thing that just happened. It did add a bit of background lore on the virus and Marcus was kinda added to the overall lore, but is quickly forgotten because Spencer exists. Ultimately, it just really didn't add much to the overall story or characters or anything. Suikoden 4&5 and Silent Hill: Origins similarly added nothing to their respective series. Anyway! To counter that, we have some incredible prequels, Like Metal Gear Solid 3, Star Wars: Knights of the Old Republic 1&2, The Chronicles of Riddick: Escape from Butcher Bay and Shadow of the Colossus.
In the end, both spin-offs and prequels can be great additions to their respective series and spin-offs/side stories are the way to go when your plot reaches it's natural conclusion but you want more money. Just don't hire Resident Evil's writers to do it.