I think this is part of a challenge many TV series face. They have a plan for a season or two, they execute really well and become a big hit, and then the writers/producers are stuck with the conundrum of having to stretch out the series indefinitely. It's only when then screw up (or just plain run out of ideas) and ratings fall that they get to write a "finale". I speculate this is why so many series seem to really struggle with their wrap ups.
Yeah, that makes sense to me, and it's not just confined to television -- for example, comic books are stuck with characters that, if popular, continue to live for decades (superman and batman are 75+ years old? X-Men is 50 or so?) and thus end up being recycled and rewritten so often that they are no longer the same characters.
I admire series that can kind of "call it earlier" and bring a graceful and sensible end to things, but the tie-in with money doesn't help, as the studios don't want to lose a cash cow.
That was part of the brilliance of BB IMO, the story arc seemed well though out from beginning to end, and the end came regardless of how high the ratings were at the time. There were no filler seasons designed primarily to stretch it out while the going was good.
I'm glad the studio was willing to support the artist, in this case. I know they scripted Season #2 pretty darn tightly (from Gilligan's comments), but they had a more organic approach in the last seasons. Gus, for example, was never supposed to be the "Big Bad" but the actor was just so darn good and his character a favorite that they went with him. (And as far as that goes, Jesse wasn't supposed to make it through Season #1.)
I felt like "Six Feet Under," while meandering a bit in the middle had a pretty clear idea of the end of the series -- who lived, who died, who was with who, I'm pretty sure that last 15 minutes was in their heads early on.
"Lost" was a little worse, I think they could have ditched a season's worth of episodes in there to make the series tighter, but things got strung out.
The "mini-series" format is actually pretty cool. It's worked really well for AHS and has jumpstarted the idea of a true "ensemble cast" who will tell different stories and portray different characters. it also allows an idea to just run its course before being closed out, and the writers/producers/actors don't get bored because they can change tone and setting and approach from season to season.
it might actually bring life into the TV format, since the studio is really only committing to one season. And they can add another if it works out. And the writers can still tell an entire story.
What I see sometimes is writers trying to "guess" how much time they have to tell their story, and either adding filler to pad it while not committing to too much, or suddenly rushing to wrap things up when they realize they're out of time.