The Inciting Incident that unearths the book was an Earthquake that uncovers the vault containing it. We know that the book can influence things so that its evil can be unleashed, such as keeping Danny from interrupting its recording, so we can't be sure that it wasn't the book that unearthed itself.
Previous films have shown that possession by Deadites can spread through infection, with bodily fluids like blood and saliva carrying it like a virus. Bridget's doom is a Foregone Conclusion from the moment we see Deadite Ellie scratch her with a tattoo needle already dipped in her own blood, and Deadite Bridget makes sure to both vomit blood on Danny and stab him in the heart with a knife soaked in her blood. But what about Beth and Kassie? Beth has an open wound on her hand that was improperly bandaged, and by the end of the film, the two are covered in Deadite blood—what are the odds neither of them got it in their mouths, eyes, ears, or wounds? Imagine if Kassie escaped with Beth only for her aunt to become possessed later...
We know that Ellie and her two eldest kids wind up mulched in the climax, but we never see what becomes of the other victims. For all we know, they got Deadited and are running amok by the end of the film.
They clearly did, they were shown to become deadites near the end of the movie.
The demonic force is now running free out in the world... and Ash, possibly the only person who could truly stop it, is apparently stuck in the 1920s.
It's mentioned at the beginning of the movie that the building is scheduled for demolition. There are still Deadites left in the building at the end of the movie that could easily infect the demolition crew when they arrive, and combined with the one that followed Jessica out, we could be seeing the beginnings of a mass outbreak.
The film's confirmation that there are multiple demon-summoning books within the film series canon clears up inconsistencies in the appearance and mechanics of the Deadites between entries- they're being summoned from different books, presumably with differentiated incantations, with different traits tied to each tome.
