The way I see it, is that they're trying to create a show for girls that has the same longevity as Super Sentai (Power Rangers), and I mean, that show's been going on since 1975 and has 38 different iterations with no end in sight. (look at all these red punks http://fc04.deviantart.net/fs70/i/2011/090/5/0/super_sentai_35th_anniversary_by_blakehunter-d3cwj7c.jpg )
Now, I'm no Super Sentai expert, but I'm pretty sure after 35 years you start to re-use your ideas.
In that sense, I have no problem with Pretty Cure lasting till the ends of time. Toei doesn't expect all their viewers to keep up with every season which is why they switch it up every year, don't do sequels, and I think its why it's lasted as long as it has.
Sailor Moon for example has its own set of characters that viewers are (ideally) supposed to watch 5 years of. And unlike American TV, it's VERY rare for a show to have re-runs. So if you're 5 years old and finally allowed to watch TV in 1995 when the SuperS arc was airing, you wouldn't have a clue what was going on or why these characters are the way they are.
But pretty cure, you could just jump in wherever and it'd be your own special season. Even if your Pretty Cure phase lasts over a year, you wont get bored of the characters either and you get a whole new collection of toys to buy. It's actually toy marketing genius
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The only reason I have for Precure to end, it would be to liberate Toei Animation's creative team and let them work on something else. Toei's pretty much the masters of magical girl anime, yet their stuck feeding this behemoth of a franchise. I wouldn't mind so much if Toei didn't have this habit of only doing 1 shoujo series at a time.
Currently they're animating: One Piece, Toriko, Driland, Saint Seiya Omega, and Kyousougiga and Precure. With collaborations to make a Marvel cartoon and that Ladybug show on the way. And presumably the new Sailor Moon.
so yeah. I'd love for Toei to do something new for their girls demographic while also keeping the precure name going