Episode Duscae and the Platinum Demo aren't the same kinds of demos most developers release. Usually, demos come out around the time the full game comes out as well. So people can sample the game before choosing to buy it or not. Usually, the process of optimization has been done or is nearly complete.

In the case of Final Fantasy XV, the demos came out long before the game reached its final development phase, or in the case of the Platinum Demo, was put together very quickly, and isn't taken from an actual finished part of the game. 

The purpose of the Episode Duscae was to show what they had planned and get feedback from the fans, about the gameplay systems and controls. It was never created as a showcase of the game's technical performance since it hadn't yet reached the optimization and debugging phase yet, which happens at the end of development.

When you release a piece of software to consumers, even a demo, people are going to make judgements of its quality. It's concerning for some to see that something that drops to 15fps was deemed fit to be showcased, even as a demo. People worry about the precedent that sets for when the final release rolls around in a few months, especially given that plenty of games have been released in the last few years with significant performance problems. Final releases running poorly has become disappointingly routine.

Maybe its all unfounded worry and the final game runs just fine, but I can certainly understand why some prospective buyers would be concerned.

I get it too. I can't say I'm not at all worried. I don't like the jaggies, and the framerate drops. If they'd stick with the 900p resolution from Episode Duscae, but implement a better AA solution and manage to get the framerate to a steady 30, I'd be fine. I think the game looks really good even in 900p.

But I also understand that there's a lot of time left to optimize the game and iron out those aspects before release. So, to me, I feel there's a lot of overreaction going on. Some people are even calling the graphics ugly, which is ludicrous to me. It's no Witcher 3, mostly due to performance and lower resolution, but it's still a very beautiful game. 

so you would be fine if the entire game would be limited and dowgraded just so it would preform right?