I'll go against the grain and say that I'm personally excited for the update. Maybe it's because I didn't play the first three titles and that makes me interested in the story update, or maybe I just wasn't expecting anything more than that. A lot of info was leaked a week or two ago, and I didn't think there would be anything significantly more than that. Can't wait for a new radio station, the song selection in FH5 is hit or miss, so it'll be great to have something else to listen to.
Also try to look at this from a business point of view. Part of the team is working on the next expansion, part of it is probably in pre-production stage for the next game. I think there is just a small team working on the playlists and medium-sized additions to the game (theme of the month, new story chapters etc) while trying to allocate new (or old-new) cars between different series. Can the playlists be better? Yes. But they are still much better than extremely samey playlists in FH4. And the upcoming update will already have more than most of the previous ones: new radio and new story chapters.
I will agree that the absence of the new-to-forza cars is a bit disappointing, but it wouldn't blend well with the theme of the month. Still, giving us one brand-new modern car as a series reward would have worked, as if saying "Now we have relived the history of the series, here is a taste of what the future holds". Still, I'm ok with what's offered, it's not like I don't have anything else to do in the game apart from the playlist.
Also, as other mentioned, compare this to the anniversaries of other games. Don't think many of them have been better than this. And no matter how hard we want something special for this occasion, a game anniversary can't get bigger than an expansion. And it was clear we are not getting an expansion this time.
