Something to remember is that multiplayer is a huge part of the Halo fanbase. Tons of people got into the series because they played against friends in Combat Evolved and Halo 2. Friends screwing around in multiplayer is even how the series Red vs Blue started. Casual players going through the campaign aren't going to notice just how deeply the changes have hurt the multiplayer experience. 343's games have had huge bug problems and tons of missing features when compared to Bungie's games, which is crazy when you realize that they already had a blueprint for what to do. There's also been a "dumbing down" of multiplayer that the more competitive fans dislike. It used to be that weapons spawned at specific points of maps and the best Halo players knew the location and interval of these spawns. Halo 4 also introduced a lot of features common in Call of Duty multiplayer that made it much easier for less competitive players to do well. Since Halo's competitive multiplayer was fairly unique before this the changes were not welcome among hardcore fans.
That's not to say there weren't people dissatisfied with the campaign. Personally I hate the changes to the lore. In the Bungie games the Forerunners were a mysterious ancient civilization and the Flood were an intergalactic parasite. In the 343 games, humans were just as powerful as the Forerunners until the Forerunners turned themselves into gray dust that became the Flood. It's an incredibly stupid retcon that didn't need to happen. Then there's all the bs they put Cortana through which I could spend all day complaining about.
If you liked the campaign of 4 and 5, and you're fine with them trampling on Cortana's character, you'll probably enjoy Infinite. But the multiplayer of Infinite was an insult to long term and hardcore fans of the series. It may very well be the death of Halo.