Calling it fanfiction is just the latent frustration from where Halo 4 took the story 4 years ago. I get how that could seem like a cop out, but that's just the remaining summary of how I feel about it so long afterwards.
As for the long winded explanation, here goes.
In my opinion (you're welcome to have your own) the most compelling part of the story of Halo was how, in spite of Humanity being incredibly advanced 500 years in the future and being capable of spaceflight, colonization, the SPARTAN program, etc. we were proven to be a fragile species, easily broken and pushed back by the Covenant.
Desperately fighting extinction was a theme that played heavily in the first 3 games, and even more so in Halo Reach. (One of the best campaigns, imo) Everything came to rest on John's shoulders. Humanity likely would have lost the war without him turning the tides.
Contrast that with where 343 has taken the story since. Here's a bullet list of the things that bother me:
- Spartan IVs - a dime a dozen! Super soldiers are now in abundant supply.
- Humanity flipped the tables in a span of 6 years. Technology so advanced, the Infinity literally rams a covenant cruiser, splitting it in 2. This type of advancement is unrealistic, even if we were reverse engineering Forerunner tech.
- Killing the Forerunner mystery. Remember the first time you played Halo 1, and being intrigued by who built Halo? Some things are better off left as a mystery. Even by the end of 3, we knew so little about the Forerunner other than that they built these marvelous structures. I didn't really want the ins-and-outs of the Forerunner explained. One of those things were reality didn't live up to expectation.
- Ancient humanity. This was the most egregious to me. Why did ancient humans need to be spacefaring in order to progress the story? I feel like 343 only added this to give the Forerunners (Didact) a reason to hate humanity, by injecting this ancient forgotten struggle between our two races. How? Well, it gets explained away with devolution! It felt forced, unnecessary, and frankly like poor writing. Plus it just doesn't make sense. How were the forerunner able to manipulate DNA to such an extent as to knock human evolution down a few rungs, but they couldn't reverse engineer a cure for the flood? Or manipulate their own DNA to make themselves immune to flood infection?
Well, there you have it. That's why I hate the direction of the story. Sorry for the wall of text.