One of fandom’s qualms with Iron Man 3 is why Tony Stark was forced to go it alone. Keep in mind that this was the first Marvel movie to come out in the aftermath of The Avengers, so it naturally raised the question: where were the other Avengers? Where was S.H.I.E.L.D.? Granted, the whole ethos of Shane Black’s sequel is basically that GIF of Judy Garland singing, “I don’t caaaaaaare!”, but fans still wanted to know the answer, which wasn’t unreasonable. If you’re going to build an interconnected universe, than trying to send Iron Man back out on a solo adventure raises the question of what exactly happened to that interconnected universe.
Black Widow solves this problem not only by taking place after Captain America: The Winter Soldier, so no more S.H.I.E.L.D., but also by taking place after Captain America: Civil War, which effectively ends the Avengers until they reunite during Avengers: Endgame. Natasha is on her own because the Sokovia Accords effectively disbanded the Avengers. No one can come to her aid because most of her allies are on the run from the government.
This also ties in nicely to the theme of Black Widow, which is that Natasha is without any family. The Avengers were her found-family and the covert-family she had when she was a child is also gone, but she’s drawn back into helping the latter when her little “sister” Yelena calls her back into action. So while Natasha has lost one family due to the events of Civil War, she has the chance to put another back together.
The prequel route also serves Marvel well here since they know where they can slot this story without affecting the larger continuity. On the one hand, it’s a bummer that Natasha is already dead from the events of Endgame, but it does keep the character’s arc firmly locked in and simplifies where exactly they can tell this story. The only other reasonable place for a Natasha prequel would be about her extraction from life as an assassin-for-hire and into S.H.I.E.L.D., but it’s clear that Marvel wanted a prequel that would also set up Yelena’s introduction into the larger MCU, so Black Widow has to handle both characters.
While some will undoubtedly have qualms with Black Widow (myself included), from a continuity perspective, the film at least clearly answers where the Avengers are, which is that they’re either locked up in the Raft (waiting to be rescued by Captain America), they’re on the run like Natasha, they’re simply out of the game like Iron Man (Robert Downey Jr.), or they’re out of this galaxy like Thor (Chris Hemsworth) and Hulk (Mark Ruffalo). Everyone is accounted for, and while it’s not like Natasha Romanoff ever needs help, at least Black Widow answers why no other Avengers are coming to her aid in her long-awaited solo story.