Quote: "you mean forever?"
The Bonanza is limited, so no, not forever.
Quote: "Even people who allready had the base game bought it "
That is right. (I'm one of them)
Quote: "Bonanza is DEFINITELY worth it, buy it while you can!"
Good advice.
The FREE -> UPGRADE route doesn't give you all the models of the full version.
The Bonanza does, and you get 8 model packs too! (plus the video tutorials on DVD)
All for only $10 more than the free version's build upgrade, so not getting the Bonanza now while it is still available would be a mistake in my opinion.