Today, 343 has finally addressed those questions, with Sean Baron (Head of Halo Infinite Live Service) revealing on IGN Live that players had only managed to glitch into an "80% complete" version of local campaign co-op:
"We of course were working on [local campaign co-op] for many, many months... years. There are a lot of challenges there from a technical perspective, and what people were able to glitch into is probably 80% [complete]. But, the amount of work for us to get from an 80% quality to a 100% quality is significant. And the amount of effort that would take right now is not something that we can commit to, and I'm not even sure how long it would take to be honest."
Baron then went on to explain that if 343 Industries was to implement local campaign co-op in Halo Infinite, it would be required to support all platforms, and therefore the challenge becomes even harder:
"There are things where, even if we have just a few crashes — I'm not sure what platform those players were playing on, because we would have to support all platforms — that's not maybe even something we can even get through our certification passes."
It's still disappointing that we're not getting local campaign co-op in Halo Infinite, then, but at least we've got a proper, in-depth response about why the feature was cancelled... we just wish it could have been pushed over the line!
Of course, online campaign co-op is definitely still part of the Halo Infinite experience, and is being rolled out today as part of the Winter Update alongside the highly anticipated Forge Mode and lots of new multiplayer content.