Okay so I have NOT been keeping my finger on the pulse of Halo's story for awhile. I think Reach is the best campaign,
and ODST had the best atmosphere and music.
But from what I can gather this is a sorta soft reboot? 343 apparently shit the bed in Halo 5 with Cortana turning out to be bad the entire time, and this one resets her character and humans are at war with the Covenant again and its Master Chief and Cortana 2.0 just doing Halo 1 again. I guess there's nowhere else to go? I think its fitting that my two favorite Halo games are the two that have nothing to do with Master Chief. Maybe its time 343 branched out. Also the Spartans are back, somehow, despite all dying at Reach except MC?
Okay, there's a lot to digest there, so taking it point by point:
-Cortana wasn't "bad the entire time." After Halo 4, fragments of her got sucked into the Domain, where she was able to gain control of both it, and the Warden Eternal, and by extension, the Guardians. Now, you can say that her shift from H4 to H5 is jarring, and I wouldn't say you're wrong, but I didn't really have a problem with it per se (I can explain why, but that would be a post on its own), But TL, DR, Cortana wasn't "always evil," and H5 never makes any such claim.
-Humanity isn't at war with the Covenant, they're at war with the Banished. From a gameplay standpoint, there's little difference, from a lore standpoint, a lot. I actually like the Banished as an idea, but...
-So far of what I've played, Halo Infinite really is a soft reboot, and an awkward one at that. It isn't just trying to circumvent H5, it's trying to recapture H1, to the point where some plot points are practically identical.
-Your understanding of the Spartans is way out of date. John was indeed sold as "the last Spartan," but that hasn't been true for awhile. Basically, there's a few Spartan-IIs beide him that are alive, such as Blue Team, Red Team, and Grey Team. The Spartan-IIIs are almost all dead bar a handful, and there's quite a few Spartan-IVs around. Basically, John's part of "the old guard" of Spartan-IIs, and effectively part of a dying breed, but he hasn't been "the last Spartan" for ages.
-Final point, 343 did branch out. In Halo 5. Only a lot of people didn't like that, so now we're down to nostalgic pandering.