So what kind of absolute managerial, budgetary, or bureaucratic screwupathon happened over there to result in this absolute flacid reception of what seemed like a big fat golden goose?!
The foundation of Halo Infinite appeared to be solid, which is why it received very positive reviews over the launch period. The problem is that they completely failed to build on that success with much needed bug fixes and content. Fast-forward to now, and it has almost been a year since launch, and very little meaningful content has been added, the the biggest bugs are still present. Hence the outrage.
We may never know for sure what went on behind the scenes, but my best guess is a combination of engine troubles, a lack of clear direction for the franchise, and poor management/leadership which just exasperated both problems.
Halo Infinite uses a new proprietary engine, that was developed in-house by 343. The problem though is that Microsoft heavily relies on contractors with 18-month limited contracts, and they rarely hire those contractors into permanent staff. What I guess this meant for the engine development, is that there were relatively few experienced engineers, supported by an army of contractors on an 18-month long revolving door. My guess is that this led to a significant amount of technical debt, which 343 is currently battling, to get new content out.
The other big issue was just 343 wasting time. Halo Infinite had the longest development period of any Halo game, yet it launched with comparatively little content - so what happened? Well, 343 has been determined to put their own mark on the franchise since they took over, which is why Halo 4 and 5 look the way they do. For Halo Infinite, there is evidence of 343 trying to turn the game into a Hero Shooter, like Overwatch, or a class-based shooter, like Battlefield, before finally realising that people who like Halo, wanted to play Halo, I guess. My other personal belief is that 343 wasted a significant amount of time on a "Halo 6" that was supposed to be released at some point in between Halo 5 and Halo Infinite, but it was scrapped after the poor reception towards Halo 5, and the more positive direction that Halo Wars 2 had taken.
Basically, everything that could go wrong, did go wrong - and they are now 3-for-3 (343, heh) on poorly received Halo games under their stewardship.
Something has got to give.