"While I'm confident that a solo Black Widow film would be financially profitable. I don't think this would be a good idea. It fits nowhere in the pipeline. Creating a suitable villain would be difficult, given that the scope of her character and abilities already overlaps heavily with the MCU's Captain America. Hydra is gone, and the coming villains are mostly otherwordly. This Black Widow is a powerful henchperson. She has no leadership skills. She does not have her own agenda aside from assisting those she feels have just causes (even if she doesn't agree!). Like it or not, her actions are dictated by the men around her - because her job is being an agent of espionage. As a recurring supporting character, she helps keeps the web of the MCU stable by being a constant. If anything, after watching Winter Soldier/Civil War, its even more clear that she functions better as a supporting character. Plot lines and character development are critical, and Marvel Studios shouldn't waste 2 hours of screentime unless they have long term developments for Black Widow and any supporting characters in this movie, such as making her director of SHIELD or something, which she can't because she's not an American citizen (lol).
Aside from that, Scarlett Johannson, as part of the first Avengers lineup is aging. None of the 7 are headlining any MCU movies for the next 4 years, and I'm sure this has been calculated.
I'm all for female representation and I think the way to go is with a blank slate character like Captain Marvel or Wasp who answers to nobody but themselves. Plus, they can do more interesting things than just martial arts and firing stun guns. Unless Black Widow's gonna rip the infinity stones from everyone and be the universe's representative to defeat Thanos, I'm not interested."
(re: 'Captain America' Directors Say a Black Widow Movie Is a 'No-Brainer' and They're Absolutely Right)