this ignores a lot of things outside the game itself.
1. We don't know what data they have that would say either implementation is or is not worth the effort (no a forum thread does not compete with actual use numbers)
2. we don't know what they have tried and decided to scrap (example per the vid, would be weapon customization/upgrade)
These could be fair reasons, but the reason given for lack of multiplayer Elites was because the story is "a Master Chief and Spartan story" even though the lore makes it clear Spartans and Elites are allies that train in multiplayer simulations together. Even with your reasons 1 and 2 which are fair, that doesn't mean there's no solutions. They could be solved post-launch as they continue to add content to the game.
3. Harmless cosmetics is actually more than that given their effect not only on hitbox and weapon balances but also the development of new UI, new animations (given elites are not the same as a spartan), a comparable cosmetic system, and the integration of those with their progression plans. There are more but you get my point. Its never just "harmless cosmetics" when it comes to new models
While your first two reasons are fair, this one is setting a very
low bar considering they were able to pull this off with technology and resources from 2004 and 2007. 343 Industries is a competent developer studio, surely they deserve better expectations. In fact, they should have even more time to implement these things post launch given that they'll be adding new content same game for 10 years unlike in 2004 when they pulled it off in a tight 2-3 year dev cycle.
My overall point though with my comment is none of us case say the majority of Halo fans or players wants A, B or C. Why? Cause halo fans exist both during the bungie era and the post bungie era. There is no majority interest. Its a melting pot of interests where the only shared opinion in mass is that we like halo.
That would imply people from the post-bungie era explicitly don't want bungie era elements in the new games. It doesn't need to be framed as a contest of "bungie fans vs 343 fans" when both can be treated. Dual Wielding restricted to campaign (even as a skull if it's that hard to balance) and custom games keeps those fans happy, and other fans are unoffended by it and can try it out if curious. Elites in multiplayer keep those fans happy, and other fans are unoffended by it and can try it out if curious. It doesn't have to be a trade off, it can be an additonal option.