Well, they'll still be in stores in the secondary market for quite some time.
The cavemen theme... I think the problem is, there's only so much you can do with that. It wasn't creative enough to really capture the imagination. More women and children might have helped. Or instead of stickers for the cave drawings, there could have been a small stencil and a couple of markers so that kids could draw their own cave paintings on. The sets were cool but just not imaginative enough, a little bit cranked out.
Bolingsbroke, I agree that continuations of themes would be a better approach, though maybe not with releases twice a year. It's not as feasible since with playmobil, almost everything involves new molds, unlike lego. Plus as rhalius said, quality would suffer. Less thought would go into the production of each set. But if they just did that approach and released updates once a year, or even every two years, I'd be thrilled. They used to update themes, rather than just discontinuing them. The only one they have done that with in a long time is the Agents theme.