You're kind of mis-representing the stats. Yes, on an objective $$ level, it grossed half of the first one.
However, "half of its audience" - what do you mean by audience? Fans of the franchise?
There were a lot of people who saw the Force Awakens who probably weren't intending on seeing another Star Wars film afterwards. Force Awakens was a historic, cultural, once-in-a-generation event. They contributed to those box office numbers, yet I wouldn't describe them as "the franchise's audience".
Also, consider repeat viewings. I know it's a small minority but it's not insignificant. Because (as i said) it was a historic event, there were strong core SW fans who saw the film multiple times. (A NEW STAR WARS MOVIE! gotta watch it over and over! this is amazing!!!) - well five films in 4 years later, that incredible uniqueness of a new Star Wars film, is no longer.
If a person watched Force Awakens five times, but only watched TLJ and TROS once or twice, does that count as "losing half your audience", no it doesn't.
You could equally say, the Star Wars franchise lost HALF its audience between The Force Awakens and Rogue One the following year.
It's a pretty meaningless claim and one that's not very true. The fact that Rise of Skywalker is level with Rogue One kind of shows you that the series is pretty stable and the core audience is still there - even if it hasn't picked up many "new fans" who now want to watch all the new films post-TFA.