: Yes, difficulty, but also simple matters like having enough vehicles and
: having enough things for four players to do. The levels were not designed
: for more than two players. For it to be a good experience, the levels and
: probably the game engine would have to be changed to accommodate four
: players. That's a remake, not a revision.

I guarantee you the engine will not have a problem with four players in campaign or even more. It handles multiple players in co-op, it handles a bunch of players in multiplayer, it handles multiple AI on three different sides, it can handle four players. I can't think of any point in the game where there aren't enough vehicles for four people for more than a minute, the amount of time it takes to find and hijack a Covenant vehicle. As for being a good experience, as I said I don't care. Not having it be possible closes doors that don't need to be closed.

: No, I'm not making excuses, and no, these are the kinds of things someone
: would say who grasps the importance of level design and balance in games,
: and how difficult those things are to create. I also have an idea of how
: difficult it is to make an engine accommodate something it was not
: designed to accommodate.

Level design and balance? On Legendary, especially if you turn any skulls on, Halo 2 is broken in that regard. Four players would do a lot to un-break it.
It sounds like you're making excuses because you're finding ways to be or act grateful for something being missing that I know you and I and a lot of other people would enjoy.
(and to be clear, yeah, it's not that big of a deal; we can still have a weighty discussion about it)