It's up to the developer. For example, Forza Motorsport 4 runs at native 4K with a locked 60 fps. Given Turn 10 Studio's history, when they say locked 60 fps, they really mean it. The Halo, Battlefield and Call of Duty games will almost certainly be 60 fps too. There'll also be games which offer the option between High IQ/30 and lower IQ/60. You're right, of course, that a majority of games will target 30 fps. There's good reason for that too - studios have done double blind tests where casual gamers always prefer flashy visuals over higher frame rates. They are so used to 30 fps - or even worse, 24 fps from movies - that it doesn't bother them.

However, with Xbox One X, it's not just 30 fps. It is the first console to support variable refresh, this is massive leap forward for console gaming. We've known this in the PC land as Gsync for Nvidia or Freesync for AMD as you point out but somehow seem to miss that Xbox One X supports this too. Of course, the Xbox supports Freesync, we should see this release in future TVs (Freesync is an open standard adopted by HDMI). This means you are no longer tied to either 30 fps or 60 fps. Most console games actually run between 30 fps and 40 fps, and are capped to the lowest common denominator in the stress points. For the rest of the game, there's a lot of wasted potential. With Xbox One X and a Freesync TV, this potential will no longer be wasted.

Around 45 fps is pretty good - I try to configure around the 50-55 fps mark, and when it dips down to early-40s I really don't notice with Freesync (on my PC). I'm sure you'll remember how The Hobbit movies looked silky smooth at 48 fps. So, a variable refresh synced 45 fps would be a great step forward.