Why were they not allowed to take it back to the design board when the design fundamentally changed?

I feel like they were, effectively. There is no reason for them to not reuse assets they created from before flying was added back in after X-mas 2016, that was still 2 years before launch. 2 years is enough to develop a game, especially if the assets already exist.

That's where the other development issues at the studio come in.

The most common anecdote relayed to me by current and former BioWare employees was this: A group of developers are in a meeting. They’re debating some creative decision, like the mechanics of flying or the lore behind the Scar alien race. Some people disagree on the fundamentals. And then, rather than someone stepping up and making a decision about how to proceed, the meeting would end with no real verdict, leaving everything in flux. “That would just happen over and over,” said one Anthem developer. “Stuff would take a year or two to figure out because no one really wanted to make a call on it.”

That's not a publisher/owner issue, that's a developer issue.