Then shouldn't they have just made an open world halo a spin off and kept the same engine for the fps halos?
My guess is they're changing the direction of Halo and transforming it into an open-world (or shared world?) and the Halo engine isn't ideal for that. Let's see!
If I remember correctly halo 4 and 5 ran on the same engine and halo 2 anniversary also had good in game engine,so why not use those and then just change only the art style?
If this is halo 6,then why the change to the in game engine?And why the change to old armor?I don't see why it being Halo 6 has anything to do with the game engine. This may come as a shock, but there have been multiple game engines over the past Halo games. Also they mentioned it was a new armor design, not old armor specifically, so it's probably just influenced more by the older games like the rest of the art style appears to be.
The problem is exactly that Halo 4 and 5 ran on the same engine. Why would they continue to use an engine that's old by this point when they could make a new one which works even better? Also, open world doesn't mean it won't be an fps. It might not even be open world, simply "more open" than Halo 4 and 5. If you've played Halo 1-3 you should be able to see that Halo 4/5 are more linear in their campaign play styles than the original trilogy, so with the return to the classic art style, they may be trying to move back towards the openness of the original games as well.
You suggest making this one a spin-off, but really Halo 5 would have been better suited as a spin-off since only 3/15 missions followed Master Chief, the main character of the series.