Well, Killzone 2 and WATCH_DOGS were still totally different situations. WATCH_DOGS truly did get a downgrade later on (meaning they showed it being played, and then the next bits continued to get visually worse and worse). It also doesn't apply to Halo because this is the first time we've actually seen it being played (even though the footage of Chief and that guy talking was running on the engine, but just speaking on like-for-like situations).
In truth, there aren't that many situations where games have seen considerable downgrades (there are actually only a handful and usually come from Ubisoft :P). Displaying a target render (usually involving offline prerendered footage) and showing true engine/gameplay (followed up with a considerable quality degradation) are usually lumped together as one entity. I know the average gamer does't know the difference (and it is on the dev either way), but that distinction is necessary in a discussion like this.
Feel like I have to mention the first trailer announcement from E3 2018 was also 'in-engine', and had at least one running shot with a Warthog traversing the open world. You can draw a comparison from that, and honestly, it's a rather bad downgrade.
Halo Infinite really needs some work to be considered 'next-gen'.