It ruins an established character
Star Trek: Discovery and later Star Trek: Section 31 really tried to make Michelle Yoeh's Phillipa Georgiou a big deal. Part of that had to do with Yeoh's own star being bigger than at any other point in her career. And this was a woman who starred in Crouching Tiger, Hidden Dragon. A world-renowned phenom in 2000. Yet, I would argue that she's more popular and more in demand now than ever.
So of course, Star Trek and their corporate overlords wanted to get Yeoh into a major project. The original idea was a brand new show, which would essentially be the same story. Yeoh would return as Phillipa and lead Section 31 coward. The cast was likely to be much different, featuring more characters from the Discovery show that got left behind after the season three time-jump, but it would've been the same concept.
Lack of interest, and a lack of funds, prompted the company to shift focus and instead make Section 31 into a film. That decision made people think the change was a good thing, and not a desperate attempt to continue to get Yeoh into something else. That forcing of the project, and the unmitigated disaster the film was ended up making Phillipa Georgiou no longer a character people cared about. She wasn't that popular to begin with and whatever goodwill she had left was destroyed by a shoddy film.
Potentially cool new characters will likely never be seen again
When a project of this magnitude fails, the future they were building to falls apart with it. So often a film or a series will be produced with the idea of continuing on well past the current status quo. Shows like The Walking Dead, CW's Arrowverse and so many other franchises were able to thrive and survive for years despite a constant change in actors and characters. Star Trek is no different in that regard.
In just about every new series or film that has come out since Star Trek: Discovery, there has either been a spinoff based off of a prior show or a continuation of some degree from a prior series. Discovery turned into Strange New Worlds, while The Next Generation and Voyager continued in Picard and Prodigy.
Even Starfleet Academy, the new show coming to Paramount+, will feature characters that were seen in other series. That's the goal here, create new characters that will hopefully pop up in other Star Trek-branded properties at some point. Be it as a cameo, a continuation, or a spinoff. Knowing that the Section 31 film failed as much as it did, the idea that we'll see any of these new characters again seems almost nill.
Sure, the complaint about them being one-dimensional and boring is fair, but a further exploration of the characters may have fixed that. A possibility that is all but unlikely at this point.