Each new entry in a franchise is contagious against old entries in two ways
1. It typically carves out new canon about its universe, and
2. Closes old loose ends
Canon when added haphazardly can:
A. retroactively create in-universe plot-holes (if X existed in the universe, then Y happening in the originals made no sense, they would have known about it),
B. remove magic and mystery from an existing universe (famously in Star Wars’ case, quite literally, because now “The Force” is a bunch of little biological creatures) or
C. otherwise trivialize what were previously key moments (someone who was portrayed as a big bad previously was later just shown to be some powerless, boring low-level lackey who was irrelevant)
Additionally, loose end patching is infectious to older entries of a story:
A. When an older work presents a mystery, when you have a solution in a later work and that solution is lame, it taints the original by the knowledge that the loose end is, forever after, a broken promise by the writers.
B. It typically crowds out the potential of better solutions later (typically in film, franchises have to reboot to backtrack on canon),
C. prevents the solution from being the potentially better “that’s just where the story ends, use your imagination for what happens next!”, and
D. makes it retroactively hard as a fan to justify recommending the series to new fans, because if they feel passionate too, you know there’s no light at the end of the tunnel for incoming fans, since you know how it ends and know that it’s not rewarding.
I enjoyed watching the best show of all time. (Of course I did!) Up until Firefly got canceled, anyway, which was enormously frustrating. But at least I got to keep the sense of mystery and the potential of big reveals.
Then they made Serenity. Which I enjoyed, don't get me wrong. But I had to do some mental gymnastics to treat it as something other than a continuation. I had to think of it as its own thing, and then I could be okay with treating it as a good movie.
But it wrapped up several loose ends up a wholly unsatisfying way. If that was what the original series was leading up to, then I could no longer care when re-watching or just thinking about the series. It wasn't so much wrapping up loose ends as snipping them off. It was a 90° turn into a different story. So I treated it that way, and it was really nice when my kids were old enough that we could watch the series together and accept that it terminated abruptly at the last created episode.
1. It typically carves out new canon about its universe, and
2. Closes old loose ends
Canon when added haphazardly can:
A. retroactively create in-universe plot-holes (if X existed in the universe, then Y happening in the originals made no sense, they would have known about it),
B. remove magic and mystery from an existing universe (famously in Star Wars’ case, quite literally, because now “The Force” is a bunch of little biological creatures) or
C. otherwise trivialize what were previously key moments (someone who was portrayed as a big bad previously was later just shown to be some powerless, boring low-level lackey who was irrelevant)
Additionally, loose end patching is infectious to older entries of a story:
A. When an older work presents a mystery, when you have a solution in a later work and that solution is lame, it taints the original by the knowledge that the loose end is, forever after, a broken promise by the writers.
B. It typically crowds out the potential of better solutions later (typically in film, franchises have to reboot to backtrack on canon),
C. prevents the solution from being the potentially better “that’s just where the story ends, use your imagination for what happens next!”, and
D. makes it retroactively hard as a fan to justify recommending the series to new fans, because if they feel passionate too, you know there’s no light at the end of the tunnel for incoming fans, since you know how it ends and know that it’s not rewarding.