As someone who played halo 1,2,3,4 Reach, ODST and part of 5 they all feel the same to me.
So its bizarre to go into the web and see halo fans screeching about the vastly differences of halo 3 and 4 lmao
I think that feeling is true of any franchise you're only casually interested in compared to people who've invested a lot more time and passion into. In a perfect world all the mainline games in a franchise SHOULD feel fairly similar on the surface so that a player can jump in a decade later and be able to reacquaint themselves before learning about what makes this entry specific.
Just using an example franchise, look at Mario Kart. The core gameplay of driving around tracks, getting items, drifting to get speed boosts has remaining consistent from SMK on the SNES to MK8D on the Switch. However the slight change in physics between games, the different item sets, the introduction of more complicated character stats, the introduction of alternate vehicle options or the ability to customize your vehicle, the introduction of more vehicles besides karts, the introduction of underwater, gliding, and anti-gravity make each of the games distinct from each other even if the core loop is still the same. However despite all of that MK hasn't been perfect, and people rightfully bitched about how bad the battle mode was in MK8, so much so that Nintendo made it a big part of the Deluxe version, fixing it and making it arguably the most robust battle mode in the series.
Halo has that in the form of changes in physics, the addition of armor abilities, the sprint button, the ability to dual wield weapons, the introduction of new enemy types like the brutes and protheans along with their unique weaponry, the introduction of every weapon being able to scope, the ability to hijack vehicles, the ability to order a squad around, and the grapple hook all radically change up how the campaign and multiplayer matches play and are designed. Halo 5's big bone head moment was cutting local campaign split screen, and it was a feature they promised for Infinite and then cut.
Imagine a world where we didn't get MK8D and got Mario Kart Switch instead, and leading up to launch Nintendo was like "we heard you on the battle mode, we will make sure a robust battle mode is a part of Mario Kart going forward again", and then at launch "battle mode wasn't ready and will be added in later" and then a year later Nintendo said "battle mode has been cut", people would rightfully be pretty pissed. That's what's happening with Halo Infinite.