They should remake halo 5 with halo 3's subtleness or maybe halo 5 anniversary will be toned down to halo 3's style not that I want to wait that long but its better than nothing. I think Genesis is fine with its style but maybe have some of the floating stuff rise up as you approach but then due to them being dormant for so long they struggle to rise and some of them sort of don't rise all the way where they should so they are leaning and crooked. I would love to see some fan art of halo 5 scenes redone in the old art style. Perfectly worded by HiddenXperia on youtube. https://youtu.be/slqVQ-kVQmw
I don’t think they would because there’s no business case to do so. It takes a lot of time and effort to make a game and re-releasing it so soon just for new art would not make much sense for their bottom line.
The "problem" (note spelt with a small p) is that there are a load of threads on the forums "they should do this, they should do that, etc" and you know what? If they did there would be posts "why on earth have they done that, I hate it".
tuhin94 is right (not many times you see people write that, just joking tuhin) makes no sense to do that for H5, maybe who knows for future games.
It is hard to explain having all the different architecture and stuff then going back to a simple art style the way people want in the sequel as far as continuity you know? That is one of the reasons I think it should be redesigned/remade. Kind of like where did the tech that was used on reach go afterward? I get it could be explained somehow but still. I know they more than likely wont change anything but it would be nice lore wise and continuity wise.
Halo 6 needs to distinguish itself visually as much as it can from Halo 5. So remaking Halo 5 to look more like 6 is anticipated to be, is a bad idea.

Although I do like the idea of 'downgrading' graphics in general. That is to say, less focus on polycounts or details, more emphasis on stylised visuals. Lo-fi visuals has been a trend for indie games for years now (high quality pixel art).
I agree that the art style didn't make sense when compared to older games. The Forerunner structures looked brand new and polished, like a bunch of Forerunner janitors just cleaned up the joint before Blue Team arrived. It would have been better to do it the way you said (rusted out and faulty components). However, Halo 5 is Halo 5 and I don't see 343 going back to change the art style alone when they are focusing on other projects that could accomplish the same goal.