Erm, Horizons has taken nearly two years and still isn't complete.
Plus (tell me if I am wrong) Frontier still hasn't said when it will be.
With some 300 devs iirc, Frontier will hardly have taken
every Elite dev and moved them onto their other two or three games. Horizon however has had the peculiar "season" model of pay first, get the content later. Meaning they've in all likelyhood already made most of the money they can by selling the addon. We know that they still plan atmospheric landings, space legs etc.. It's likely enough then, that the smallest number of Elite devs is actually working on Horizons stuff and the larger part on post-Horizon addons - which can earn them fresh money.
Hopefully!
Otherwise coming Elite addons may suffer from the same problem as Horizons: Nearly no content at launch, but functional additions sprinkled over a span of two years instead. We'll hopefully get a glimpse at what's coming out of Frontier Con in October.
Oh, Jurassic World! Park building with dinosaurs? Sounds intriguing. Of course, also because of the dream of eventually go big game hunting for a proc-gen space T-Rex in Elite.
Edit: I'm very happy Frontier gets to work on such an iconic IP. Here's hoping you people at Frontier will do it justice!