Yeah, it would simplify things, in a lot of the attractive streamliney ways the MCU seems to favor. But I think it’d be a mistake to switch one plotline out for the other. 616-wise, the two romantic interests do vastly different things for Natasha’s narrative.
The Soviet side of the Black Widow/Winter Soldier thing is continuity padding, the sort of weird shared universe coincidence connection that makes, say, Captain Britain Spider-man’s old college rooomate. They have a footnoted affair in the past but the meat of their story is in the present, in Bucky’s redemptive journey back to herodom and Natasha’s unique ability to help him through it. He also provides something none of her other tights-wearing beaus have really given her— an unspoken understanding of her darker origins. Matt and Clint, in the comics, both accepted her, but they didn’t, and couldn’t know where she came from, and that played into the dynamic of their relationship. In MCU, though, Clint’s been transmogrified into a special ops sort that already offers that close, grim partnership.
While Winter Soldier was this retroactive plotline invented to power present continuity stories, Alexi is her past writ large. His refusal to stay dead is metaphorical as much as it is comics!!!11 He is Natasha’s Uncle Ben, her dead parents in a Gotham gutter, the grief that really drove her to become the Black Widow. And while they’ve moved away from that story the past few years in favor of “more brainwashing”, I think that was kind of a mistake. In any case, Alexi hasn’t gone away. Because Natasha’s origin has two parts. There’s the grief she rode into battle, that let her become the ultimate weapon, the ultimate tool. And then there’s the revelation that that grief was manufactured to control her— that it’s something she has to free herself from, no matter how deeply or genuinely felt. Alexi comes back, over and over, so that Natasha can defeat him, over and over.
You can’t really do that with Bucky, especially since he’s got his own manpain enabling duties over in Captain America. Plus there’s this unfair bias against female characters who don’t “stand on their own”— people are really quick to dismiss characters like She-Hulk as cheap knock-offs. And while I think that’s stupid, I also think it would probably be a mistake to take such an integral figure from Natasha’s past, from her story, and switch him to Mr. Already Appearing in These Films.
That said, I think you could make a good Black Widow movie without going into either of these storylines much at all! The Yelena Belova plotline hints a lot at both melancholy that drives Natasha and the brutal spy organization that trained her: all of the origin beats without the unwieldy flashbacks or dudely complications. (You could even adapt Clint into the Matt Murdock role, no trouble.)