Most of the time books are better than the movie or TV show.
https://www.barnesandnoble.com/blog/6-reasons-the-book-is-almost-always-better-than-the-movie/The link gives the six reasons why its mostly true via Barnes and Noble. I think as media consumption continues to grow, this is not nearly as true as maybe 30 years ago.
Where the money would be is those who are able to take a source material and successfully adapt it.
Not everyone successfully adapts the source material well.
Since we're comic oriented, I'll stick to comic books that have been adapted.
League of Extraordinary Gentlemen. It was made into a movie released in 2003. The concept for the film was brilliant. And for the comic book. However, it was poorly adapted and the source material was awful. This would be one of those cases where someone should have stepped and done a better job with the material.
The Invisible man was completely a disgusting character in the book. The movie cleaned him up as best they could. The bottom line is nobody in a theater would have rooted for him as a 'hero type'.
The studio added Dorian Grey and Mark Twain (his inclusion was because the movie was deemed too British). Neither character was in the book and sadly they were the most interesting to me in the film. The film flopped but I did buy the book it was based on to learn, that I'll never blindly buy a book based on a movie or TV show ever again. I'm not kidding about how put off due to the Invisible man.
The thing with books is the material is not limited. And as society changes some subject material is less acceptable but its a book so ... it gets a pass? I don't know.
Umbrella Academy - the TV show was adapted really well. There were a lot of things that had too change to make it work and fit a small budget for a streaming service and studio. Number 4/Klaus the most changed because it wasn't practical. It was also would not have been wise for Number 5 to do certain thing. And they couldn't really pull off the look for Number7/Vanya. I looked into the book and no, I'm not reading that.
As streaming is becoming a bigger thing and Hollywood trying to find the next best thing, having to have a block buster as many weekends in a row or what not. Audiences are fickle.
As Ice noted in the other thread, movies not cutting it are immediately removed from the theater. Making room for the next film with hopes by Hollywood they've got something.
Look at Xmen: Dark Phoenix, it got pulled after the first week because it tanked. Now, here is a case where the source material wasn't horrible. It was a classic tale in the Xmen book but it was so poorly adapted it was just utter junk.
Anyway, these are just examples, not necessarily talking points. Just showing in my view how the book is not always better.
Discuss if you want.