As much as it's bothered me how much complexity Valve has added to TF2 over the years, potentially intimidating to would-be new players, I think fundamentally the game is still very much the same and new players don't really need to know about every single weapon unlock in order to play. With the exception of a handful of new weapon categories — flare guns, bows, charging shields — most of what they're up against is just slightly different versions of the same basic weapon types, with no particularly novel strategies for not getting killed by them.
In contrast, adding new entire classes is likely to dramatically affect what new players need to know going in. Nine is already straining the boundaries of information overload; I can't imagine what 18 will mean for people.