I must commend the op for his homage to the historical side of things, although "a huge step forward in human evolution" is rather farfetched. While I agree with above posters that the shotgun already fulfills that role and has reached a point of counterbalancing the sword, that the spear would be impractical, that the energy sword was originally a ceremonial weapon, and the fact that every Covie gun having an exact UNSC counterpart would be redundant and occupy space in the sandbox, I would point out that there have been precedents for including out of place weapons. 343i clearly had no issue with putting a grav hammer on Midnight (as an easter egg or trophy?).
Brutes in H2 stockpiled and favored human shotguns so if Spartans using spears seems out of place, 343i could just give them to a Covie "heretical" sect which uses human weapons (like the ones in Reach firefight).
I was reminded of the GCPD trophy display room from Arkham Knight. At one point in the story, Batman has to return to this place to retrieve a REC gun. Do recall that there was a H3 ad with a museum with some Spikers on display. A Halo Infinite level featuring a section taking place in an UNSC museum (Penguin's museum in Arkham City is arguably the best detailed museum in any video game) would allow for an older model spear to be retrieved and used (not mandatory to beat the level though) for that segment without feeling too out of place (considering the CQC situations a la The Library).
Also, this is just me being subjective, but in the Dark Souls series, spears are just as viable as other more commonly used weapons, with some of the rarer ones having unique abilities and movesets. If implemented properly, the UNSC spear (or even Covie spear) would make for a welcome asset.
What I find to be ironic is that the UNSC was situated in a futuristic setting, but they're still using bullets (excepting the Spartan Laser, Railgun, and some other stuff found in the novels). Modern tanks IRL have faster movement and are equipped with modern tech while the Scorpion feels sluggish (even with it's turning), very outdated and out of place. Very few innovations have been made to it since it's inception. To me, the Scorpion has always felt underwhelming compared to the Wraith, despite it's higher firepower, accuracy, and increased durability.
I would also caution against justifying gameplay changes with canon. Reducing the AR's clip size and the severe nerfing of the magnum in H3 was clearly intended with MP in mind and they should have just said as much, but Bungie decided to retcon in it by saying, iirc, that the original AR had jamming issues. While I'm not too bitter about the AR change, the magnum should have had a scope (to accentuate it's remarkable accuracy), a larger clip size, and a faster firing rate; it was practically worthless in campaign, almost completely sidelined by the BR (whereas the magnum was still somewhat viable in H2). At least ODST/Reach/H4 managed to make the magnum relevant again.