I speculate that when then did CEA they really did consider doing this, but they looked at the time and effort that would be needed to do it *well* and decided that it wasn't the appropriate place for 343i to focus at that time. As a studio, they were still quite new and focused very much on their first real baby, Halo 4. While Certain Affinity can handle a lot of the aesthetic upconversion/HD/graphics work, I'm guessing that the netcode is much more something that 343i controls.

CEA was something of a stopgap ... give the audience something to play with while H4 was being created. Keep the effort reasonably light as such. Don't reinvent the netcode.

I'm sure that at time, too, there were internal discussions about the next-gen console (XBOne), dedicated servers, etc. and it was evident that to really do this right would be to leverage those next-gen resources.

Lastly, since H4 has been released, I assume 343i has really had to re-evaluate how to do multiplayer. It's evident that some of their gambits in H4 haven't worked as desired and it looks like H5 is really taking a look back, trying to leverage the best experiences of H1-H4, and really try to determine what a Halo MP experience should be. Simply put, this is a big moment of reckoning, and I imaging that a lot of this thinking and plumbing being built to work in this New World dedicated servers proved to be a better fit for H1 than trying to shove in a peer-to-peer model.