I don't get this "we're targeting" line. Isn't this a technical design decision that should have been committed to along time ago?
Hitting a constant 60 fps at all times is very hard. You are dealing with tons of assets, that all need to come together in a playable level with AI, graphics shaders etc and they all have to precisely hit 60 fps. You have so many variables, that you can't predict ahead of time that for example an asset of 10 000 polygons exact with 4x 512x512 pixel textures and is used by your physics engine, will give you 60fps in a level. Its constant tweaking to see where you can scale back in for example graphical fidelity to provide you with 60 fps game-play (its much easier to scale back then scale up, if you have knowledge of how models and art works then you know what I mean) Also levels when they are developed are always changing, decisions are being made, new assets are created and put in.
You for example add in a field of grass, but that gives you bad performance because of the amount of polygons it draws, so you scale back on that and then another guy comes in and places in a detailed statue that is a main part of the story, so to hit that 60 fps you need to remove the grass and find some more recourse efficient ways to do the grass. or during game-play people are finding the grass useful, so the guy that did the giant statue needs to optimize that. It is a battle!
rope is being pulled from all directions.
Its like trying to hit an ant on a moving freight train with nothing but a pebble.
Its super hard and the only think you can do just under deliver in terms of graphical fidelity. Use the current gen consoles as target but render it on a higher resolution.
I mean you can do rough estimates but its impossible to give players a constant 60 fps experience while pushing graphics on a new console and decide that as a hard target ahead of times.
edit:
You are also dealing with different devkits when developing, specs changing (getting 8 instead of 4 gb of gddr5) diffrent versions of the SDK. its impossible to commit to a hard target, thats why they say, they are
targeting 60fps.