I mentioned this in the FFXV Forums (where it got mostly ignored in the recent days), where I tried to rationalize the whole production, just not as long as this one. And it's quite simple.
First you have to look at the KH series, which is one whole convoluted mess of a story (I love KH), the story is a complete mess where you have to play a prequel released 5 years later to understand the event of the first game for example. However with enough understanding of the overall story, it's has a deep and rich core at it's base. However that is also why Versus/XV could not see it's full potential.
Nomura has a very twisted way of expressing his visions. However, considering the nature of the KH series, creativity was very limited. And looking at KH in a certain perspective, it's very dark. Which is where I think Versus was born, a world with no restraints. I'm sure he had a story set in mind when he started the project, however his mind was elsewhere all the time. Not just because of FFXIII, but he did direct numerous KH games from 2006 to 2014. He went back and forth all the time and because he went out to create those KH games envisioning new plots, he vision continuously evolved. So when he came to work on Versus, he brought with him many different ideas.
Which was the biggest downfall of the project. Development with no real forward progress. They kept reworking on the same things.
Then came the switch to the PS4. A new generation opened new possibilities. New technology allowed for new story telling elements, things that could not be done before. This was their first experience at that. What happens when you are given more choices? Well some try to take all of them. It's not impossible, but it would take a lot of time to complete. So by adding new things, other have to be changed to fit. It's like a puzzle piece, you change add a completely different piece, it's not going to fit, you have change the surrounding pieces to accommodate it. And thus the overall picture continuously changed.
However, the problem does not end there. Nomura was still working on other projects. His head was still not in the right place. So that's where Tabata came in, a man with no concurrent responsibilities. Someone who could sit down get focus just on one project.
So is Nomura to blame? Personally no. No one is really to blame, just overall mismanagement. Starting off at the end of the PS2 era heading into PS3, they tackled new territories, in which they hit many walls. Then coming into the PS4 era, essentially the same thing happened again. If Nomura was to blame for everything, he wouldn't be where he is now. I think his abilities are why he is Directing FFVIIR, because he has a knack for being able to create a great immersive game. However, Versus was just to big to be just a side project alongside many others. So why FFVIIR? I think it's because FFVII already has a existing plot, it's a solid plot already laid out. It will be easier to add things in than a puzzle not even completed. By taking an already existing plot and possibly adding elements that Nomura strived for in Versus, it could result in something amazing.
Or perhaps he'll just become the puppet of KH for eternity.