There will certainly still be Horrors in Magic's future, however. Horrors generally have a different character from Demons, and serve a different role. Where Demons are winged, horned humanoids, Horrors are usually out-and-out monsters with little or no physical similarity to humankind. They are nightmares come alive. There are plenty of examples of Horrors; here are just a few.
An interesting question: are there Horror cards printed between Fifth Edition and Onslaught that might have been concepted as Demons, if policies had been otherwise? Invasion's Devouring Strossus looks to have been a direct descendant of Lord of the Pit. And Odyssey's Wayward Angel might have had the right concept to have been a "Creature - Angel Demon" rather than an Angel Horror. Can you think of others?