1 They caught up to the "current manga arc" back before the previous anime-only-but-canon arc.  There just wasn't enough time and the Bleach-style of "more filler!" ended up being pretty disastrous, for that series.

So, it'll end.  Given that Fairy Tail is still a top-5 manga series, I get the feeling it'll be back in a year or two.  There is still a 5th OVA coming out in October, I believe.  So, at least more Lucy fanservice.

2 I'd rather they'd go out at the top of their game, than to drive the whole thing into the ground. Too many series have worn out their welcome.

3 For instance, much as I like Strike Witches I'd be perfectly happy if the movie was the last of it. The only story remaining I'd like to see is the end of the war, and they'll never show us that one. It doesn't really seem like there's any other story to tell at this point.

4 For as much as we want to see some characters/worlds be in the mix, some stories really are just "done".  Most action series really operate this way, unless they go the "build the ENTIRE WORLD" One Piece style or do something completely different (which I'm having a hard time thinking of a reference for that).

5

DBZ was supposed to be "done" when Frieza was killed. That's when Toriyama was ready to quit, but the magazine wouldn't let him. So he did the "Androids" arc, and that really was good. And he wanted to stop yet again, but they wouldn't let him.

The "Majin Buu" arc was crummy by comparison. And Toriyama knows it. When they redid DBZ as DBK, they left that out and ended it with the Androids. Which means the series ended with Goku dead.

6 This is normally the cause of Shounen Fight Inflation issue.  Most series are started with a basic story arc but not enough world building thought past that.  (E.g. Bleach and the Soul Society arc)  So, the only way to keep any tension is to, well, make "another person that's harder to beat!". 

There are a number of ways around this issue, but it has to be done at the beginning.  Compounding the issue is that series continue in print because they get popular.  That's where the artists makes their money (and why the publishers won't let them stop, at times).  Everyone stands to gain from continuing a popular story, but most of the problems in a continuation stem from choices made before they start.  So, most of the time, there's no easy way around it.

The "Buu Arc", at its peak, had half the Galaxy destroyed.  I believe that counts for "largest enemy inflation EVER", or something.  And, yeah, it was pretty bad.  But at least Toriyama went all out.

7

Tengan Toppan Gurren Lagann has a reputation for the most extreme inflation, or so I understand it. (Let's don't turn this comment thread into a discussion of that series, please.)

Fairy Tail has on several occasions given Natsu powerups, but they were temporary. It's obvious the author was aware of the problem and was trying to avoid it.

In the Nirvana arc, Hibiki gives Lucy a ridiculously powerful spell called Urano Metria to cast against Angel, but Lucy didn't get to keep that one, either.

Over the course of the series Natsu, Wendy, Gray, and particularly Lucy have definitely increased their power (and, of course, Lucy has acquired many new keys), but not at the kind of rate that we saw in something like DBZ, and by the end, 250 episodes in, they haven't achieved the kind of totally absurd power level we saw in DBZ.  (In the Majin Buu arc, Goku and Gohan and Vegita have achieved such tremendous power levels that they have totally surpassed the Supreme Kai, who is basically God.)

I understand that in the current arc Lucy finally acquires the last two gold keys, meaning she owns all 12. I'm looking forward to finding out if that has any significance beyond the fact that it makes her the strongest summoner alive.

8 From what I know of the current manga arc, Lucy has acquired a lot of power.  Maybe.  There's apparently a lot left to be revealed, which is kind of the point, I think.

Though I know she also ends up a bit of the Fairy Tail punching bag because she gets targeted by the super powerful types.  So, I guess there's a given and take on that point.

9 "I understand that in the current arc Lucy finally acquires the last two gold keys, meaning she owns all 12. I'm looking forward to finding out if that has any significance beyond the fact that it makes her the strongest summoner alive."

She gets to make a wish and they all go flying off in different directions? *gdr*

10 It's kind of a shame, but everything comes to an end eventually.

Have you ever seen how many episodes of Sazae-san they've made? Crikey!

11

It's been in production for more than 40 years? Good Lord.

I hadn't heard of that one. It puts Detective Conan to shame, doesn't it?

12 To be fair, it's not really something that comes to the attention of those living outside Japan, but it actually has the highest viewing figures of any anime show in Japan by quite a significant degree.

13 I guess that's a good enough reason to keep on making it. But you got to wonder how they can keep going that long without becoming stale.

14 I'm sure they've repeated stories many, many times... but if it's been running for 40 years, who's going to know than the latest episode is almost exactly the same as one that ran in year four?

15 What with 3900 episodes I don't see how they could possibly avoid repetition.

16 I'm pretty sure about 15 years ago they recast most of the characters and just started remaking old episodes explicitly. 

Oh, and it still pulls down something like a 15% share each week.  Between Sazae-san, Doraemon and Chibi Mariko-chan, the weekly animation blocks actually pull down some pretty massive TV numbers in Japan.