I'm with you on understanding (and sometimes having) the desire to have a show end while it's still good.

Also, I find that if I, as a viewer, choose to stop watching a show because I feel the quality has dropped, it's hard to do so while staying active in fandom, even just at the reading-fic level, since authors quite often jump off from points of canon, and those points might well be the newish ones responsible for a person jumping ship. It's a little complicated.

Anyway, I find the short-burst style of most BBC shows create a very different dynamic. There's less canon, and it's often quite spaced out, which makes me get to the "oh god cancel this while I still have some affection for it!" problem come a lot more slowly.

but, as with everything, YMMV