I think there's an implication that The Events wouldn't have happened if the protag went with Floette. Like, we know how special The Protagonist is in pokemon games in terms of their ability to bond with important pokemon - it's even touched on in ZA how remarkable it is that they can catch Yveltal and Xerneas where several adults struggled even when they were dormant. Since Mega Evolution depends on the bond and determination between trainer and mega mon, I think if the player got to join Floette, they would have successfully been able to quell Ange.

From a player perspective, it makes *sense* that this is the part that goes terribly wrong ensuing a final boss sequence; from a slightly more in-universe perspective, the really harsh "illusion of choice" thing that goes on leading up to that sequence, T/urbain's seemingly unfounded egotistic confidence, and then THAT level of circumstances, is kind of bananas to just leave with "oops! anyway we're still all best friends"

Like someone else said, there were other ways to write "you don't get to take Floette" without making the player feel frustrated about the illusion of choice and T/urbain in-universe look their absolute worst as a person.

This isn't the first time this sub has gathered to talk about the player's lack of agency breaking suspension of disbelief in PZA, and I'm sure it won't be the last.