They should still have had a convention next year, just remember there were no movies to promote between the wait of clone wars movie - eps 7, yet they still had cons) overall i feel like fans like interacting with each other and buying merch and celeb meet n greets/signings.
There is so much content they could put in a Convention even without one particular movie)
They could have filled it with
Filoni's next project (early concept teasers, or if not bring him even to talk about Rebels)
Games (Any Battlefront dlc, whatever games are due out, apps etc)
Comics/Books
Sound and music (let us see the freaking panel, spend some good time on it)
Toys (The Vintage 2018 range etc)
Art
Panels with the Stars (now theres no big movie to promote they could extend these a bit longer for instance what was Tudyk onstage for maybe 15-20 mins? Make this now 45 mins)
Updates on how the Park is going.
Put out the freakin 4K Blurays (no matter what state despecialized, specials, bluray to 4k versions whatever edition, just bring these out and you could have a panel talking about the process people would want to see)
Announce there is more movies in the works next year, and then say we will tell you more in 2019