With an interlocking film and television franchise, you don’t have to worry about creating a whole new universe that fans will have to subscribe to. Instead, you get to add something to the main-line Star Trek series and see it grow. Fans love deep, expansive universes. It’s why after all these years there’s still such a thirst for Lord of the Rings or Star Wars content. Fans want more of that universe.
They may not want it all at once like Star Trek is currently doing, but there will more likely be a market for Star Trek for as long as there are people who still want it to exist. Growing continuity can only satiate those fans
Yes, there’s always the risk that they add something that turns off the fans. Trek fans know that’s a possibility as much as anyone. The new properties have been some of the most divisive pieces of entertainment in some time to come out of this franchise.
Though, if fans can get past Spock having a sister that was never-ever-ever mentioned before, there’s a good chance another film, that’s attached to a former television series, isn’t going to ruin anyone’s good time.