I highly doubt devs will target 60 fps versions of the same games. Or at least, that most will.
Getting as much as possible at 30fps is always what's the priority. Anyone who doesn't think Sony or any other publisher is going to use the ps4k version of exclusives in marketing is lying to themselves. On the back of these games boxes, it'll be pictures of the upgraded version, on TV and on the Internet, it'll be the upgraded version. Hell, I bet one of the ways the'll try and sell the system is to show the ps4k and the vanilla ps4 version of games running side by side.
And what about games that are already at 60fps like call of duty or battlefield? Sure battlefield will be at full 1080p now, but that's only what, a 40% pixel increase? What will they do with the rest of the power? Increase the fps past 60? No way.
Sure, some developers will just bump up the framerate. Probably a ton of smaller studios will. But a ton of developers, including first party ones, will focus on how the game looks, not how it runs. And it's going to start with games like god of war 4 that are being developed on the ps4k and then being ported down.