It took 3 and a half years to make.
FFXV is a completely different game to Versus XIII. With similarities purely used for marketing reasons.
It's a different game because they wasted most of the decade on Versus XIII and failing to get it off the ground.
It's more than just "similarities". It's the same lead character, Noctis, most of the same supporting cast, most of the same characters, locations, and even lore as initially planned.
XV is literally built out of the skeleton of Versus XIII. It may have different flesh, but the framework is still rigidly Versus XIII. It wasn't marketing reasons; it was sheer necessity to salvage nearly a decade of work without tossing it out and starting from scratch.
... Which brings me to something that's blowing my mind. The last mainline Final Fantasy game that didn't go through development hell was Final Fantasy X, back in 2001, a whopping 16 YEARS ago.
Every major Final Fantasy project since then has been mired in problems, development issues, and major setbacks. FFXII's had to quit half-way through production and the staff are quite open about what a nightmare developing it was. Final Fantasy XIII was horrendously managed and they admitted over 80% of what they developed was tossed out and all those banal sequels were made just to reuse all those wasted assets. Final Fantasy XIV was a legendary disaster that required a full-on redo from the ground up. FFXV was mired in development hell as Versus XIII and finally got momentum when they switched up directors, but for all its good qualities it's also clearly undercooked and lacks focus and clear direction, despite someone doing something with Noctis for a decade. There's a reason there's currently a survey on the menu of the game asking players what they should do, because they don't have anyone there with any strong leadership skills to blaze a path forward.
And now this.
Square Enix doesn't know how to manage their development and has so few people in charge capable of spearheading a game with razor-sharp focus and development cohesion.
It reminds me of an old interview I read on Final Fantasy IX, where they talked about how it was difficult and how they had three or more branches of Squaresoft working on the game at once, communicating mostly between the Japanese and Hawaiian branch of the teams, and as a result they had to create the "bible" of FFIX to get everyone on the same page. Before a single programmer or CG artist started on the game proper, they had to ensure the story worked, the characters all had proper character arcs, the pacing was right, the world was cohesive, and everything lined up, because they simply couldn't scramble about in the dark, throw stuff at the walls to see what sticks as they went along, or rely on lengthy surveys from players for direction. They had to figure it out in advance, and once that happened, they were able to smoothly develop the game on-time and on-budget because there was no last-minute changes, no sudden shifts, no post-launch story patches.
Square Enix needs someone with disciple and vision to do these games, because the current crop of developers mostly don't have this and we're now at a point where the majority of their mainline games this past decade are being finished by people who step in to complete the job the initial developer or director couldn't.