The problem with V1.0 is that they were aiming for PS3 using a game engine that was already designed for the PS3. The problem is that the 1.0 PC engine was broken on day 1, using techniques that may have made sense on the PS3, on PC hardware that can range from being PS2 level to present-day hardware.
In fact you can run the V1.0 benchmark on todays hardware.
Much of what was better in 1.0 is nostalgia-tainted. Yes the models were far more detailed, and would have been appropriate in a single-player game. However bring multiplayer into it, and guess what, that engine falls apart pretty darn quick.
It's actually somewhat of a miracle that ARR was pushed out in such a short time as it was. How they did this, was grinding down the detail of the V1.0 models, or abandoning parts of the 1.0 inventory.
As for "butt slider"-meme, it's just pointing out where one piece of detail was sacrificed to reduce the complexity. Most people don't care about the size of the butt, it's the fact that the the Miqo'te tails are always clipped, and the Au Ra tails are clippped, shows us why this change was a bad change. Would I have preferred a larger butt on my character? Nah, not really, but the clipping on the tail annoys me a lot and it means I pick gear that the clipping is less obvious. If every piece of gear had a proper tail cut-out I think people wouldn't even notice.
FFXV (15) uses the same game engine as XIV. But you have to realize that the XIV game came out before XV did. And you'll notice where this code base is in common because if you play FFXV PC maxed out... it's so much more buggy than FFXIV... and why is that? Because it literately outstrips the capacity of the highest end hardware. Sound familiar? That's what happened with XIV 1.0 PC using the FFXIII engine.
However you can scale back hardware requirements on XV, where XIV 1.0 you pretty much couldn't. Not unless you wanted to run it at a illegible resolution.