Interesting points, Lego, so to speak, are not "running out of ideas". They are a money making company not a museum, they repeat sets like the fire brigade and the Police Station because they sell well. And if they satisfy kids, which are there primary audience, then TLG had done their job. Also, if Lego decide to rehash or produce a similar set it means they made a overall business decision which benefited the company as a whole. Whether that brings long term or short term benefits they do it for the company.
Now to your second point, it doesn't matter where you live, and what store you visit, you can't judge the popularity and success of a set simply on your own findings. Saying "X theme isn't selling well because I always see them on shelves" isn't an accurate way of deducing set statistics. Now if various people from various countries were to come forward and state their sightings that would be different, then we may have a chance of predicting what is selling well and what isn't but one person in one toy store isn't enough.
You also mention how "Nobody Buys" Friends, DP and Elves, yet if you visit the Special Themes Forum you can see a lot of people like them..