And somehow the Avengers movie was able to survive not having Ant Man in it. The Big Plan didn't spiral out of control or anything.
Ant Man could have never come out and Civil War would have survived that great loss too.
None of that discounts the point which is that Ant-Man is carried over/delayed movie (which you're conveniently ignoring) which was meant to be dropped with Thor, Captain America, Iron Man and Hulk for The Avengers crossover. That's where the movie was supposed to "fit in" and that's what it was originally for.
You can't use them making that to justify a Black Widow movie when Ant-Man was always on the original slate with the other characters I mentioned. If you're going to say "no one was clamoring for Ant-Man so Black Widow should have been made", then you have to say "No one was clamoring for Iron Man, Cap or Thor so it should have been made" but I bet you won't