All of this is rendered moot when you take into consideration that every halo at release has been 30 fps.
Saying all Halo were 30fps is no different than calling Halo 5 a 60fps game. Halo 4 and Reach especially struggled with frame rate, with frequent drops to the low 20's, so why would Halo 5 need to be 60fps every instant?
To maintain locked 60, you'd need to be aiming for a game that can easily hit 70fps+ and that's just not possible with the Xbox One hardware whilst still having great visual fidelity and decent resolution.
I agree with a poster above saying it's a tech thread tries to invalidate real discussion on what exactly is achievable in the end. Why is a game like Forza even brought up as evidence that so and so should be able to do 1080p/60? It's a racing game. Halo should be directly comparable to other shooters ON XBOX ONE in terms of tech. And as we've seen, even the top games so far have struggled doing more than 720p/30, 900p/30, 900p/60, 1080p/30
343 is perhaps the premier studio Microsoft has. They're not incompetent, look at what they achieved with Halo 4. They have the ability and resources and knowledge to utilize the hardware more than any other developer. If they have some issue, then you know others would have the same. I mean Xbox hardware engineers are probably working with the 343 staff on the game at parts. I fully expect this is just pre-launch paranoia much like Halo 3. Halo 5 will ship, it will look and play amazing.