Hey Puzzlebox, here are some sources on game completion rates

In the last article, it says the industry average completion percentage, which is supposedly 20-25%
Here's an interesting Gamesutra article that talks a little bit about this,
http://www.gamasutra.com/view/featur..._feedback_.php . It goes into the completion rates on some of the top games the year the article was published.
http://www.1up.com/news/bioware-deta...effect-2-stats is a quick look at a single game. Bioware released some statistics about Mass Effect 2, and it showed that roughly 50% of players finished the game. This one strikes me as particularly crazy, since the game is both narratively driven, one of the most critically acclaimed games of the year and actually rewards you for completing it by allowing you to import your saves into Mass Effect 3.
Finally, it seems to be true that more people finish adventure games,
http://ps3.ign.com/articles/115/1153279p1.html . Turns out about 72% of gamers finished Heavy Rain.
While I agree that it isn't necessairly a great reason to make games easier, I think it is most likely a consideration they take into account.