I know you're going to jump around my neck. But, PERSONALLY, I think the negative reactions to this are being overstated a bit. In each and every Halo game the iconic Halo CE weapons have changed. And I have never had a problem with this.
An example that comes to mind was the case of Halo Reach, where the BR and the Covenant carbine were replaced by the DMR and the stinger rifle. At that time there were many discussions about this change. With very negative comments. But in the end they ended up liking.
I will simply wait to try them. Giving them the benefit of the doubt.
And regarding the issue of how they look physically, there are those who say they are very CoD, and they don't like that. I personally see them as futuristic within the UNSC aesthetic, and I quite like them. I think this point is already within the personal tastes of each one.
the shotgun and magnum where included from the first game. reach was an outlier, as it used the unsc army arsenal and a special covenant fleets gear. halo infinite plays with marines, so marine weapons are expected.
the magnum after halo 2 was continously refined, the shotgun was there since halo ce. the changes of the shotgun in halo was a refinement process too, not a replacement process. the sidekick fills a specialist role, while the classic magnum has a utility role.