Good question. A few reasons, in reverse priority order. First, the licensing fee is likely to be under $5000, which would be bearable (though painful) in budgetary terms. Related to that, I might consider joining the existing class action lawsuit and am not above milking that to get some extra publicity out of it.

Second, it was the original inspiration for the story. Now I was already well aware of the license issue so as soon as that key story element fell into place I knew it would have some sort of price tag attached, but sometimes you just have to trust your instincts.

Third and most important, it's because it's so universally well known that it works. I have one other piece of music I anticipate licensing in there, but if I had to for $$ reasons I'd explore alternatives.During early drafts I noted several good musical cues that would enhance or complement the story action, but while they'd still be my ideal creative selections that would probably unaffordable on my budget, and so I'm happy to work around those.

But 'Happy Birthday' is not just an expression of my taste, it's so universally known in the English-speaking world and beyond as to be a major cultural signifier. If I were to use some alternative birthday song like 'Woot woot, it's ya birthday' or something, it would be a huge distraction from the emotional content of the scenes, and turn them into something lame, at best, or comic, at worst. I listened a few of the alternative birthday songs linked elsewhere on the page, but (unsurprisingly) I forgot all of them within 60 seconds. You only get a few key moments in a film where characters are really strongly defined as individuals - you know, the lines that you can quote from a famous movie that everyone recognizes, eg 'I'm gonna make him an offer he can't refuse' or 'You talking to me?' or 'There's no place like home.' You have to keep things simple because audience connection with a character is a matter of feeling rather than thought, and if you're trying to process something else to evoke the same idea, then you're not emotionally engaged with it in the moment.

To some extent, I'm leveraging the strong emotional connotations that the song already has for in order to intensify the dramatic impact of the scenes where it is used. In the same vein, those scenes also involve a birthday cake as opposed to a birthday apple pie or indeed a birthday steak - cake is a cliche but cliches are useful for drawing attention to the abnormal things about the situation for dramatic effect. I want to make sure that people remember those scenes where it's used.

Mind, my issue with Warner/Chappell here is that I really don't think Happy Birthday is actually eligible for copyright protection. If I was going for some other emotional situation that just had to have a particular piece of music whose copyright status wasn't in doubt then that wouldn't bother me, although there are very few pieces of music that are so iconic that they can't easily be substituted for - the only one I can think of is the Bridal Chorus from Wagner's Lohengrin, better known as 'Here comes the bride...' for use at weddings, or maybe Taps for military funerals, but even that is a specifically American thing.