Well, ない means non-existence, and なくなる means for something to become non-existent, so it's like "the buses will all become non-existent," or "there won't be any buses." I think it's fine to think about it as ない+なる. That's how I think of it, anyway.
Incidentally, I found it interesting that there's an entry for 無くなる in the dictionary. I'd thought that only 亡くなる (to die) would be in there. It says that it's the same origin as 無くなる (makes sense, but it still doesn't seem like there needs to be an entry for 無くなる).
「なくなる」の意味や使い方 Weblio辞書