I think I'm one of the few who thought Lucario should, and had a good chance to, return. From my perspective, there were a couple of things in his favor that I think people overlooked.
1.) He's still important to the series overall. He continued to be promoted as this special pokemon long after the fourth generation had passed, which is particularly noticeable in X and Y, where Lucario actually has a degree of plot significance. That's pretty impressive for any pokemon.
2.) Why replace him? He might not be the newest thing anymore, but is that a reason to cut him out entirely? Perhaps more importantly though, who would replace him? The sixth generation didn't really introduce any kind of "Lucario" pokemon, and the last "Lucario" we had, Zoroark, was starting to get left behind. The closest thing we had to something like that was Mewtwo and his new form. Now that could work. However, when you consider the likelihood that the Pokemon series would expand to have five character slots, you realize that we could just add Mewtwo back onto the roster without replacing anyone. And that gives us a very solid roster of Pokemon characters that makes the vast majority of audiences happy.
So, because of those reasons, I thought he had a fairly good chance of returning. I had my worries, since everyone of course said he was dead, but it seems I had nothing to worry about in the end.
