Damn, it's been a while since I've written an actual forum post... I agree with you in the sense that there are many flaws with Halo 5 as a game, but I simply feel like there's a severe lack of logic in your replies that indicate slight bias - which is understandable - but doesn't help in the long run at all. Hence I'd like to break it down, and on the off chance you do reply I'd be happy to discuss.
"Don't fix it if it ain't broke" - Just because something is working adequately well for you, does not mean it is the game that others want to play. Never changing anything is going to lead to stagnation, and although all the changes 343 (Or really, any game developer) makes are in pursuit of a better gaming experience - whether it be to cater to newer tastes or to update and refresh old game mechanics - it's only natural, and unfortunate, that they aren't able to please everyone. Nonetheless, never changing anything = never improving. That's how you get bored, and that's how you slowly kill off your player population.
"WHO WAS THE MAIN VILLAIN!?!?" - I thought this was quite obvious, considering Cortana acts against the Master Chief. Furthermore, as Locke, your main objective in the climax of the game is to thwart her plans. Nonetheless, questioning who exactly was the 'bad guy' was probably very much one of the themes and ideas that 343i wanted to convey in Halo 5's story in the first place.
"Why were the covenant, prometheans and swords of sanghelios there!?" - Well, that's why you should've paid attention to the story, right? I mean, come on. While the argument could be made that some information was in books and not revealed in games, it was clear through playthroughs that the Swords of Sanghelios and the Covenant were in a civil war. And the promeatheans... Well, there were tons of Forerunner Structures. Some missions in the game took place on a Forerunner built Planet!
"I didn't even list half the things I could've listed." - List it, please. You've only said like, 3-4 things, and most of your points have been somewhat... nonsensical.
"Someone from 343 listened to the complaints about only selling one req at a time and said he'd take it to the team and get it fixed. Here we are, a year after release, AND WE STILL DON'T HAVE IT!! " - People make mistakes. The team has to prioritise their problems, too. What's more important to functionality of the game? Broken matchmaking? Or the inability to sell multiple REQ packs at once? What's going to keep players continuing to log on each day? Selling their REQ packs? Or new DLC that includes new maps? Different game modes? Yes, it's an issue that they said they would do something and they still haven't done it, but 1) We have no idea how easy it is to fix and 2) There are many, many, many more important things that they are working on.
"So you tell me, how is Halo 5 in any way not completely ruined by 343?" - I mean... It's working fine. People still play. The game is good, fitting both conventions for the Halo series as well as the modern FPS genre. It's made probably way over half a billion in profit for 343. It's got many good reviews. I'd say that means it isn't ruined.
"fight for no reason because they have no quarrel" - Well, one of them disobeyed the chain of command. The other was following it.
"watch as they fight, not talking, not doing anything until AFTER Locke is immobilized. Why didn't they do anytihng?" - Great question, one that you'd have to pick up on both teams' thoughts, as well as understand their personalities, in order to understand. Locke heavily respects the chain of command - he has put Chief's role as the saviour of humanity out of his mind and focused on him instead as the object of his mission and his orders. Chief, meanwhile is fixated on reaching Cortana. Blue Team is family - they will go wherever Chief goes, but they aren't blind dogs. All of them know they are going against their orders - and they've been taught to follow orders their entire life - and it makes them hesitant to engage (Not to mention they knew the outcome of the fight between Chief and Locke before it even happened, and all 3 were most probably ready to pounce should the other members of Osiris decide not to look on as bystanders.) Meanwhile, the other members of Osiris still held heavy respect for Chief and his team. They were saviours of humanity - it was unthinkable for them to betray the UNSC - much less for Osiris to actually attack them.
If I remember correctly, the soldiers in the Meridian mission weren't marines. As for the Marines in Warzone, ignoring the fact that the servers were probably heavily taxed and 343 felt like Marine voice was less integral to gameplay that many other game systems, you know that canonically they are simulations as well, right?
"Why is 343 trying to appeal to younger audiences?" - That is not their only target audience, but it only makes sense to want kids to play your game as they're the ones who're gonna play the next ones and the next ones while series veterans move on.
"What makes halo so good is the adult content Bungie put in it" - I don't remember any adult content, and I honestly wouldn't say violence - which was the reason previous games had a higher rating - was the reason previous games excelled either.
"The Arbiter had no reason to be there!" - Well, other than the fact that that was his home planet, of which he was trying to lead.
"Cortana and the guardians make no sense whatsoever!" - Well, it does make sense, even if in some ways I personally feel the story was in some ways contrived. Pay attention, or if you aren't gonna play the game again, ask away. I'd be happy to explain what you don't understand.
"That's not even Cortana anymore, it's like a different version of her!" - Debatable, but the specifics are up to the story's creators.
"I don't want to see halo 6 unless you promise me it's good" - I'm sure it's gonna be friggin awesome. Unfortunately, just like every other game in existence, it probably won't suit every player's taste.