Kingsglaive was decided to be made a movie from the start of the project; Tabata knew from the get go the invasion was never gonna be in the game. If they had built invasion elements for modern XV and then scrapped it, they'd have utilized it for Chapter 14. Instead, Chapter 14 Insomnia at launch was literally all they had developed for FFXV. The other set pieces from Versus were repurposed for Kingsglaive. Scenes like the ship crashing into a building above Noctis while they're on the streets echo a similar scene in Kingsglaive. The shot of Noctis staring out the car, watching the ships flying is replaced with a similar scene in Kingsglaive where Nyx is watching the landing of Imperials.
They simply utilized the 25% of Versus Nomura had completed to fluff up early trailers. Even if it was on PS3, the few CGI cutscenes Nomura would have made (and did make, considering how many cutscenes were shown off in abridged format in old Versus trailers) would look pretty enough for FFXV's "PS4 footage". The game was made in chunks with several chapters being worked on at once concurrently, and due to changes in the story or just the fact that it was different teams, they'd have these completed chunks that were incompatible with each other and required retooling, leading to a similar problem XIII had wherein the teams wanting the engine retooled for various areas broke the game until it was super linear to make up for the demands. FFVersus seemed like they were going the cutscene route like XIII, and so they had a plethora of footage and set pieces to fluff up for their Ebony tech demo for E32013. Only in 2015 it seems did we start getting exclusively XV footage without Versus content thrown in. Kingsglaive's CGI took three years to develop and polish.
The anime is an example of content that was cut later into development and thus required an inexpensive method of telling the story. They had to cut out the DotF stuff and thus focused on the bro trip; then early on, like the invasion, realized that they couldn't tell the bros story and the Niflheim conclusion, so we got DLC announced, presumably with story to lead up into it. Then they had to cut out this story to lead into it because the game was facing development problems with its main story, so it becomes Brotherhood anime. Like DotF and other DLC addition, Kingsglaive started off as a piece of cut content from XV before getting fleshed out as its own unique piece of content. As they did not want to reveal Kingsglaive until before the game's launch in Uncovered, they kept using Versus footage of the invasion as the game's events are triggered by the invasion.
The complaints about the invasion not being in the game are why we have the jarring, awful Kingsglaive clip show in the main game when before, the slow lead up to the newspaper scene was more impactful. Every time they realized they had to cut something out, if they cut it out early, it became refined content. If it was closer to development's end, it became other content like the anime or Platinum Demo. If they had a last minute cancellation of development for a specific piece of content, it became cheap material like the novel.The prologue, Parting Ways, definitely feels like they planned to have it tie in to Kingsglaive more and showcase Noct's normal life, but given they were struggling with the end and already had a decent beginning, fluff like Parting Ways and Brotherhood couldn't be given proper TLC in the game. Same with DotF. Each time they cut content, they got ambitious and tried to do more afterwards, which blew up in their face with the final DLC.