I figured it was for plot reasons. And upon rereading, it does make more sense - for some reason, I thought it was Fawkes that was coming, not another phoenix. My original reading was that Fawkes was somehow going to spitefully reject a chance to destroy Azkaban because Harry had missed his chance, which seemed absurd - knowing that Harry was giving up his chance at a phoenix of his own, and not his mission, annoys me much less. I'm still not sure if I like the new 85 more than the old, but it's closer now.

Also, I'm probably being optimistic about how easy the writing of it would be, but my first impression is that it should be possible to make a version of it where Harry's resolution is the rejection.