Congratulations, you have an argument against a Black Widow movie. Now let me tell you why that argument doesn't really work.

Okay, but she wasn't really well known to the general public before the movies, so it's probably best to keep her in a supporting role. It's just too risky otherwise. 

*points to Guardians of the Galaxy*

*also points to Antman, which absolutely no one was clamoring for*

Okay, but she's too well known now. She's been in several movies and is very important in the team movies. That's enough, isn't it? Give other characters a chance.

*points to three Iron Man movies, soon-to-be three Captain America movies, soon-to-be three Thor movies, and two unsuccessful Hulk movies*

*also points to the fact that in one of those movies an important, character developing scene for her got cut because it "slowed things down" and that will always be considered an acceptable loss until there's a movie about her and her development specifically*

Okay, but now you're getting a Captain Marvel movie and a Wonder Woman movie.

*points to twenty million superhero movies starring dudes*

*also points out that lady characters are not interchangeable and that a Black Widow movie would not be the same kind of movie as a Captain Marvel one*

Okay, but she doesn't have any powers, and that's not very interesting for a superhero movie.

*points to Batman and his gazillion movies*

*also points to James Bond, Bourne movies, and other popular spy thriller franchises*

Okay, but they follow a strict timeline, so it wouldn't make sense to do another origin movie for an established character.

*points to her current successful comic run that has nothing to do with her origins but still somehow manages to find interesting stories to tell about her, like, how do they even do it*