A happy ending appeals to a larger audience. It is what most of us instinctively want: to rejoice at the end.
Why do you think most titles aimed at the general audience feature a happy ending? Because said general audience wants Lucy and Kouta to reunite, so they can go 'Awww...' before the cast roll appears.
Additionally, studios are well aware that the taste of the general western market may differ from that of Japan. Its business we're talking about, and the best way to be successful, on a grand scale, in the west, is to throw in a cliché they can appreciate.
It is exactly the 'could' that is the issue here. The audience is allowed, if not encouraged, to harbour of the two's possible reunion. If the silhouette was shown to be in agony, or bleeding, or something among those lines, I'd say, yeah, they sacrificed Lucy for a decent ending. There is, however, an absence of any such thing. I don't thing there is much moot as to whether the scene was put there to reassure viewers.
As for figures, it was a reference to merchandise in general, of which there is a moderate amount, considering that Elfen Lied is more of a fan title, and can not be considered mainstream when looking at the general visual entertainment consumer.
You have watched the anime because your view is broad enough to allow for an inclusion of more serious themes. As for the average person, who wants nothing more than popcorn and glamour, the fact that the show may be said (internet reviews, magazines, peers, etc.) to have a bitter ending, may very well be a crucial factor when making a decision about watching the series.
Let us just look at the film industry: If you want to sell out, make something predictable, cool and glamorous, with mono-dimensioned characters.
The sad fact is that people are rarely forgiving when you go and actually kill their beloved characters, however professional the matter in which you did it.
But yeah, I admit that my opinion is especially subjective, given my preferences.
